Warning: Anecdote, not data, etc: I just started playing with Draw Something last weekend, and I do have to say, it's really powerful: Getting a message a friend of you (!) has drawn something(!) and you don't know what(!) and you can score points(!) by guessing is really powerful, and wants you to open the message (and, thus, reply, as that is built in). I also like how they combined free users and paying users, to grow the network and make revenue. I don't know if they will exist in a year, but in a few weeks they have grown faster than Instagram in 1.5 years (and THAT was a really quick growth in itself!).