There's actually a UIUC alum center in SF - http://www.sfbayillini.org/. The last estimate I heard was that there are somewhere around 40,000-50,000 UIUC graduates in the Bay Area.

I grew up in Chicago, graduated from UIUC, and I live in SF and work in Silicon Valley. When I moved here, I didn't need to make any new friends if I didn't want to - when I got here, my entire friend group and extended UIUC social network was already here. Shit, my girlfriend (also from UIUC) works at a startup in FiDi where both founders are from UIUC and there are many alums.

It's just too damn easy to move here for us.