The interesting thing is that most of those people weren't actually pals when working on P2P stuff, but they were all working on the same stuff. Zuckerburg and Parker didn't meet until Facebook was growing quickly. Scour/Red Swoosh, to my knowledge, had no major connection with the other projects.

I view it more like an instance of synchronicity, where a lot of bright young people looked at the world independently and decided that this was the space they wanted to be working on. And then when that space didn't pan out they went their separate ways, but the fact that they were bright & energetic meant that the successor projects became huge.

(I wonder if a similar effect explains the General Magic, Paypal, and Justin.TV mafias.)