The fastest growth is occurring on Tumblr, Snapchat and Instagram. Unfortunately, I am not that familiar with Tumblr but considering its active user base grew by over 120% this year, maybe I'd want to be. Instagram seems to mainly be popular with teenagers and Snapchat has gotten so popular, it rejected a $3 billion offer from Facebook.
Its hard to predict what will happen to social media sites in the future as we can't predict their next moves, but I firmly believe Snapchat is becoming increasingly popular especially with its new updates. I feel there might also be another form of Whatsapp, which could be detrimental for Facebook's recent investment.