This article talks about how people make fewer friends as they get older. It mostly sounds depressing: parents who make friends only when their kids do, busy people who deduct points from friends if they're late, retirees who find themselves totally friendless.
More interestingly, though:
"...the three conditions that sociologists since the 1950s have considered crucial to making close friends: proximity; repeated, unplanned interactions; and a setting that encourages people to let their guard down and confide in each other..."
Seems simple enough. Why not create that environment after college too?