you're giving full copyright to the social media website you're posting on. If someone wanted to buy a licence to use this - whatever it might be - the discussion would be between the social media platform and the licensee. the original author of the work would not have any stake in that theoretical situation.
If you were wondering which specific case I'm referring to, ForHackernews linked to the wiki article. there is a small note on the licensing issue at the end there.
https://news.ycombinator.com/item?id=42509955
From what I remember, he had gotten a WB offer - which ultimately didn't pan out because a licensing agreeming couldn't occur. He'd have had to rewrite the story off-reddit for them to be able to license it. And that never happened.
(Well, he did rewrite it - but probably took too long, so the window of opportunity had already closed and it was never made into an actual movie)
A social media site typically takes a soft licence allowing it to store and reproduce your content (which is needed to be able to function), and maybe use it in marketing. Some go a little further, but please show me one mainstream site that takes over all your (copy)rights when you post?
What I feel comfortable stating is however that we have precident for the exact scenario the person I responded to (wanting royalties for a storyline they posted on a social media website) and this precident showed that are least the lawyers of WB were of the opinion that a rewrite outside of any social media platform was necessary.
... do you mean precedent of a scifi premise from social media being turned into a movie? or the precedent of a piece of media using a piss-tank's levels as a means of communication?