There have been other paid participation social networks such as Tsu and ChangeTip. It doesn't appear to be a very compelling feature, because in reality you can't pay very much to users. Each user is only worth on average $20 in revenue per year to Facebook for example. The problem remains how to differentiate with some feature users really need that they can't already get on their existing social networks. Steem attempts to pay $1000s to 2% of the users and the rest get nearly nothing, in attempt to get users to incorrectly assess their earning potential on Steem. But I believe if users only came for the promise of $1000s and instead got only pennies, they leave if there isn't some other compelling feature that other social networks don't have.