But after the April cataclysm, many returned to the fold just as quickly - B2B jokily morphed into an acronym for 'back to banking' and B2C stood for 'back to consulting'.
Unfortunately for investment banks, however, not everyone was so opportunistic about their decision to leave investment banking - for the more serious breed of defector, it was an important statement.
The trend has weighty implications for the human resources departments of investment banks struggling to find top-quality staff.
Initially, investment banks responded to the pre-April exodus by offering an increasingly eclectic mix of personal perks for their employees, from chauffeurs, dating and valet services to childcare. But these were unlikely to address any fundamental issues that induced staff to leave - and even when the fall-out in technology stocks began to bite, most banks used money as the chief lure to get people back.
One investment banker in his 20s who was in a highly rated technology, media and telecoms (TMT) group at a bulge-bracket bank, left to join a start-up with talk of seven-figure remuneration. 'The dot-com required a different skill set and was a great challenge. I had just got engaged and thought it would be nice to stop the banking hours and still make some money,' he says.
Within 10 months he was back, at a smaller investment bank. He was offered two-and-a-half times the cash he was paid by the bulge-bracket bank he left, as well as internal private equity funds. He says the bulge-bracket bank wanted him back and was quite annoyed that he went to a rival, but that he did not go back for 'political reasons'. The issues moving him were about lifestyle and money.
Brenlan Jenkins is another investment banker whose career has taken him from Salomon Brothers to six years as a consultant with AT Kearney and McKinsey. He joined a start-up for 18 months and then worked with Ed Annunziato's team at Wit Capital, the online investment bank now known as Wit Soundview. Differences over strategy caused him to leave and he spent 10-12 weeks in the summer looking at start-ups, incubators, private equity firms and investment banks. But ultimately, he joined Lehman Brothers, where he has been for about six weeks.
Jenkins heads Lehman's efinancialservices practice in Europe. He says: 'Lehman has the infrastructure but also gives me the freedom to work with small companies. We deliver M&A advisory work and capital raising in private markets plus private equity and generic advisory work for the companies.' It was never about the cash, but about what he thought would be a good role for him, says Jenkins.
'Had I thought this would be a bog-standard transaction-oriented business I would never have done it. This is carving out a new market and a business that needs to be built. It is another entrepreneurial business that just happens to be inside Lehman Brothers,' he says.
The desire to do something of an entrepreneurial nature was an important theme for many in their decision to defect to a dot-com. Foster Bowman, the founder and managing director of idealing.com, was an OTC derivatives trader for about eight years, four of them with the then Bankers Trust. He left Deutsche Bank two and a half months after it took over, but says the push factor for him was not actually the merger. He had started working on a business model in late 1998, in the middle of the US internet bubble.
'The Bankers Trust (BT) environment was very entrepreneurial. I was at the top of a learning curve and once I was at Deutsche I quickly realised it was going to get harder to get on the next rung of the ladder. I wanted to do something entrepreneurial,' he says. Bowman has no regrets, and idealing.com, which is entirely backed by private funding, appears to be thriving.
'It's dramatically different in terms of atmosphere. The cacophony of a trading floor is non-stop, and your day is completely reactive. Politics are a necessary evil at big institutions. You have to do it to succeed and get ahead. It's nice to be able to dwell on something else,' says Bowman.
Dealing with internal politics is one of the most often cited drawbacks in climbing the ladder at an investment bank. 'As you get nearer the top of the pyramid there are fewer jobs,' says one senior ex-investment banker.
Alasdair Warren, a former investment banker turned entrepreneur, says: 'If you are performing well you have to be conscious of those who are shooting you down - and that makes for a very stressful work environment.' Warren left his job as one of two managing directors running equity capital markets at Schroder Salomon Smith Barney in March 2000 to co-found nCoTec, the technology venture capital company.
Another senior ex-investment banker says: 'The trouble is that when you work for an investment bank everyone else is the enemy - and you don't necessarily know anyone from any other investment bank.'
They talk of weighing up the 'push' and 'pull' factors in the decision to leave. To leave at senior levels the 'pull' factors have to be very strong because you have worked very hard for a long time to get to that level of remuneration. There appears to be a consensus among many in their late 30s and early 40s that investment banking has changed substantially since the start of their careers, and these changes have ultimately been responsible for their decision to leave.
John St John, the former co-head of global equity capital markets at Lehman Brothers, is chief executive of EO, the online share distribution platform. He says: 'Investment banking used to be about providing bespoke advice to individuals and working in firms that behaved as partnerships and were involved in all aspects of a deal. You were intimately involved with many clients of the firm.' Now, he says, the integrated financial services machines where people work at very specialist levels make the job repetitive and commoditised, and these factors detract from its enjoyment.
He sees this as a phenomenon throughout the ranks of investment banking - to make a machine work more efficiently you have to specialise more and more. But for him the 'pull' factors were ultimately the decisive factor in the decision to leave.
'The internet was clearly going to revolutionise capital markets. I felt no one else was looking at Europe with the optimal business plan and it was a unique opportunity for me to leverage my skill set. I knew that no one else had yet done it better - unlike investment banking. Here we could start without any real competitor and take my career into a different space after 15 years in investment banking,' says St John.
Success, in terms of being seen to be good at what you set out to do, is terribly important to many of those senior investment bankers who have left the industry.
For Matthew Hale, who spent 17 years at BT and ran its Global Securities Finance Group at the end, BT was a 'fantastic' place to work, with a spirit that he describes as 'entrepreneurial and collaborative with a strong ethical base'. He has now established striding-edge.com, a company that invests in very early stage internet-related businesses.
'I am simply refining what I was doing at BT. But one big difference is that I no longer have the support of a huge organisation behind me. There is definitely more risk and mistakes would be more costly, but the satisfaction of succeeding is that much more as well,' says Hale.
He argues that very many people in investment banking make far more money than they deserve. 'At an investment bank you are paid for the franchise because you are there. A better test of one's ability is surviving in the real world. The challenge lies in learning to operate without the support and learning to be humble,' says Hale.
For many investment bankers who are in a privileged enough position to be able to leave with significant financial security, the chance to control one's own destiny is cited as a prime 'push' factor.
'I had a wonderful and varied career in investment banking,' says David Turnbull, who spent 20 years in investment banking, 13 of them with SSSB.
He left his position as managing director at SSSB to set up antfactory.com, the international strategic investment company. 'At antfactory we have a wide variety of people from different backgrounds with great CVs. They are the sort of people I would not have met in investment banking. It is an opportunity to build something from scratch with global reach,' says Turnbull.
He defines 'fun in the workplace' as being a mixture of the people you work with and ultimately success in what you aim to do. Although in many ways his work is quite similar to what he was doing as an investment banker, he says his role is far more operational, with the benefits of a quick decision-making process. Other investment bankers have left to use their skills in directions close to their hearts. Mehmet Golan, who worked at BT and Daiwa, left the industry and travelled for a year after getting married.
His travels led him to decide to do something commercial to bring about positive social change, and he founded drparsley.com, a company targeting a variety of socio-economic issues.
It has been going through a second funding involving sacrifices in a difficult market, but Golan has no regrets. 'I like making a lot of money as much as the next person, but I just want it in combination with other things I consider important,' he says.
As the shake-out in dot-com companies continues in the new year, the HR departments of investment banks are reluctant to comment as they watch for opportunities to woo back some of the best talent that has left. But they would be well advised to listen to those who are still determined to find a better way of doing business.
One of the best-known senior investment bankers is Ed Annunziato, who left Merrill Lynch to head the European operations of Wit Soundview. He says he looked at Wit as an ideal opportunity, with the possibility of speed in the decision-making process.
'It has given me the opportunity to use my skills as a banker and a manager of bankers, and combine it with my experience of the private equity, IPO and M&A sectors - and to build a new, more open culture in which to realise these capabilities. That is just not possible in traditional financial institutions,' says Annunziato.
Over the past year investment banking suffered a sort of collective mid-life crisis, as it became clear that many who had stayed climbing the ladder had simply not allowed themselves to think of the alternatives.
One senior convertible investment banker who dipped his toes in dot-com waters after 14 years in investment banking returned to a French investment bank after a few months last year.
He says: 'I lost some money and I came back to a slightly less well-paid job because I had time out of the market. But from a life experience point of view I have no regrets. I had never taken something from business plan to reality before.'
Alasdair Warren at nCoTec says: 'In the past few years senior people have been remarkably well paid and feeling well capitalised. There is a huge new opportunity for people who are financially smart to create value for themselves in a way that has not happened in Europe before.'
It is only when you get out of the investment banking environment, he says, that you realise how many opportunities there are out there. 'You ask yourself, why didn't I do this six or 12 months earlier? But that's life, isn't it?' says Warren.