Good question, probably still facebook and twitter but that doesn't mean others like pintrest, instagram, and google+ should be forgotton about!
I think if they can increase and give nice innovative, I think this social network will be winner for next.
Pinterest is a new star of social media site for this year - 2012. And we will see the new star in 2013.
I think Google+ is getting familiar day by day and people are also in need of something new to use it so there is a chance for Google+ to be a popular one in next year.
I think Google+ and Pinterest are becoming popular day by day and savvy marketer are getting huge traffic, popularity with these sites.
May be Pinterest......... Pinterest is changing day by day... One month ago they allows users to verify their website. Few days ago Pinterest allows their users to create their own Business page. Changing a lot.... Pinterest is now a big trend........
There seems to be a buzz going on right now for this new social networking site called rebel mouse or something like that, so I've heard. But probably nothing compared to Pinterest