As some said in the comments it might just be a seasonal thing, and with over 500 million users across the globe losing less than 10 million might not mean much. I'm also not seeing buzz about anything new that people might be drawn to. It could also be just early adopters quitting.
Long term I think social media is here to stay, but evolve, regardless of whether it will be Facebook carrying the torch or something else (even multiple players). Social media and search are going to merge as well, if nothing else then cause the search engine will become almost like a person with which you have to have a relationship (he might be called Watson), but I'm talking crazy now.