This is a relatively silly graph (and, according to HN comments, pretty inaccurate) but it's interesting to think that, as time passes, the YCombinator value proposition is slowly but surely shifting from "we'll give you a bit of money, a lot of advice, and a lot of top quality investor introductions" to "we'll plug you into the most powerful startup founder network in the world."
YC alums count the founders of startups that have achieved all levels of success, from billion-dollar Dropbox and AirBnb, to large acquisitions like Heroku, and many others. I'd wager that there isn't anyone worth knowing in the startup world that isn't within the immediate network of one or more YC alums/founders.
