One in particular is full of alumni from a certain Silicon Valley company, and it's interesting how many people we meet are on that list. I've even heard stories of people only realising who the other was when they when a list thread came up in conversation. "Oh, you're X! Ah..."
Is it something about companies that their employees believed in that lends itself to coherent post-employment communities?
Only one of my ex-employers has built such a community - and it's something to say that the mailing lists are soon about to rival the now-defunct company for longevity! It was a company that prided itself on its employee community, with Chief Morale Officers in each office, and regular social events (then, again the way we worked there, our fellow employees were our only social life!).