The thing with Harvard Facebook is that it's a predefined social circle that came online as a unit. If you go to Harvard, most of your friends and people you regularly interact with are also going to Harvard, so you have a cohesive group altogether.

With Google+, invites were more or less random, so in a group of 5 friends maybe 3 got one and 2 didn't. Were the 3 people who got invites supposed to just drop their other 2 friends? Obviously people aren't going to do that.

People do like things that are exclusive, but they want to exclude outsiders, not their friends.