I'm not sure if Graphene in specific could do it as it is now but I do think it could be upgraded to manage it. I know for sure Tauchain could do it which is why I've given so much of my attention to Tauchain. Freenet is a bit outdated (2001) technology and doesn't even handle basics of the semantic web so I would not rely on it. The technology you need is the technology behind the semantic web which Tauchain promises to have. Solid also looks good and is made by Tim Berners-Lee, designed specifically for social media.
Generally though, it's going to happen sooner or later whether we see it on Tauchain or Graphene. To me a blockchain is just a data structure. But it does not have to be rigid, you can make it flexible and Tauchain shows that, but even to some degree you see it in DPOS. The blockchain allows you to have a flow of time, a sequence, an agreed upon shared state for any specific point in time, and for certain things this is necessary, but you can use other data structures too like tangles(DAGs) or even what SAFE Network is doing, depending on what you need to do.
Steemit needs the data structure it has but you can do sidechains with it to extend it.