I don't think you can call craigslist anything like a non-profit until the story is over. Currently they are earning millions a year while the value of their business continues to skyrocket. I don't think there's anything wrong with this, I just wouldn't call it a non-profit because they are making hundreds of millions a year in (unrealized) capital gains. If one day they cash that in with an IPO, then all of this "non-profit" talk will just have been a clever marketing strategy. If they don't ever cash in, or give away the bulk of their fortune, then these guys are the next Mother Theresa. But only time will tell.
I've heard different answers to this question before but how exactly does CL make money and what is the source of these "unrealized capital gains" to which you refer?