Facebook opening up to non-students was the turning point IMHO. Myspace was big, but everybody knew it was trash so not being on it was fine. If you wanted "a profile" otherwise, you needed your own page. That took effort, so only people with something to say bothered with it. Even Twitter was still SMS based and so only for hardcore addicts.
Facebook gave everyone an effortless voice and lordy, do people talk crap.