These platforms already have a their identities and I doubt they will risk integrating a system that will drastically change user experiences. Rather, projects such as Steem, LBRY(The LBRY project is a "decentralized digital library" that seems very promising), and Breakout(A gaming multicurrency platform) will grow because thay are developed on a blockchaon and can do what these platforms do to distriute content but with rewards and incentivization systems at their core, rather than slapping a blockchain on a model that is thriving today, but will likely get outpaced in the future by better alternatives.