It's hard to say. I think Twitter would be much more likely. The reason is that everything on Twitter is public and about retweeting and following and all that. It's very similar to Steem in some ways. Facebook is setup to be private with your friends' list etc. Reddit or Twitter could try, though it'd be at least a year or two before they do I'd say. They'd probably prefer to avoid it as they'll lose money if they do.