(This is assuming that all social media sites are down for exactly one year and nothing can change that)
Lord almighty! We might have to... socialize! I must take a seat and sit down at the very thought of going outside, the very thought of seeing kids on the sidewalk PLAYING instead of taking 'sexy' photos for Facebook. There'll be the people who, recluses to the end, would just back away from the computer and play Xbox or something for a year and wait for their precious MMORPGS or Facebook or whatever, but I'm guessing the lives of majority of the people who use social media websites won't be that different. They'd be disappointed at the loss of Facebook and Twitter and whatever, yes. Angry, yes. But will it really affect them in the end? No.
Though, the loss of social media websites for one year would, as Live said, be a loss to the world of news. It's effectiveness was clearly shown during the Arab spring; what if an event of that magnitude were to happen again this year and all social media websites were down? It'd be interesting, to say the least.