Twitter, in my opinion, works best as an app. I don't even really consider it a social network - it's more of just a news feed plus.
The only thing that exists now that has a chance at overtaking Facebook is Google+, hands down. And unless Facebook can get back to it's roots, I hope it does. I'm really tired of these... non-people Facebook profiles, like all these 14 year old girls posting their pictures on "site modeling" Facebook pages.
Am I the only person who sees things on Facebook and double checks to make sure they didn't log into myspace on accident...? "/