They could. In my opinion, discontent on a platform is rarely obvious until it stabs you in the back. You can brush off the angry users on reddit and point out (correctly) that they are a small hivemind of trolls and bullies. However, that minority is preaching to a silent majority who are thinking "You're an ass, but I don't think they should have done what they did to you." But they continue to participate in the site because they are comfortable, and there aren't any viable (better) alternatives. This is just dandy, except that the user base no longer has any loyalty to the platform. When there is an alternative, a minority of more actively discontented users move, and they tell their friends. The friends tell friends. And so on. In a very short period of time, the original site is a desert, and the "alternative" is now the place to be. It could happen to Facebook, with a few more mistakes and the rise of a better alternative that could pull the teen audience.
I think facebook already is in a downward spiral. I'm 16, and most of us tend to use stuff like Twitter, Instagram, Snapchat, or Vine rather than facebook, which is probably why you see Facebook acquiring stuff like Instagram, Snapchat, and Vine. I'm coming from reddit myself, and I'm really enjoying the format of Hubski, especially the lack of obvious centralization. One thing I would like, however, is a way to see a "score" on your posts, but that's not a huge deal to me to be completely honest.