You do have a point +Ziyuan Yao
, it might just be luck that FB started out with some Ivy Universities and Colleges.
Also, considering that some of the other Social Networks of the time like Hi5, Friendster, Myspace were well established and have millions of userbase. Google is pushing the people to use G+, but having a g+ account is different from using G+. i see g+ more of a bulletin board but not a Social networking.