An excellent point, Facebook replaced MySpace which replaced everyone's Geocities, Angelfire, Tripod, etc... pages. Prior to that, there was AOL. All of those are gone or essentially gone. Something different (not necessarily better) will come along and people will flock to that.
The same goes for Twitter. I created an account a year ago and followed a couple people, one of which was Chad Ochocino. He must have tweeted 50 times a day. How can I follow that? I don't know what anyone else was posting because it was all lost in a sea of garbage from Ocho. I dumped my account after a week. There's lots of worthwhile info on Twitter, but I'm not following 5,000 people for the odd valuable tidbit. I'd rather read a news article stating that "so-and-so" broke the news on his/her Twitter feed. I don't need to know 10 seconds after it happened, unless it involves me personally. I often hear coworkers talk about Jack Campbell's Twitter feed (he's the goalie for the Windsor Spitfires). These are grown men discussing a 17-year old boy's tweets. Our society is in the crapper.