Yea, I mean, it's
extremely unlikely that Facebook will be "switched off." But, I think there are some possibilities that range from "extremely improbable" and "illegal / probably illegal" to merely "improbable."
1. Current admin declares "fake news" to be a national emergency. Demands Facebook turn over all keys to all databases. Democrats in Congress cry foul, Republicans are "concerned," until Facebook doesn't turn over the keys. Justice system gets involved, and then the debate turns from "extraordinary government overreach into free speech and privacy" into one about how a corporate is spitting in the face of American sovereignty and security, and "activist" judges, etc. Probable outcome: nothing happens because a Federal Judge in Hawaii says no. Possible Outcome: Facebook gets scared and hands over their data, tanking trust and usage.
2. Current admin orders a massive investigation into Facebook ties with Russia, throws around accusations like "spycraft" and "treason." As servers and documents are seized, Twitter et al step up recruitment and snipe staff terrified of FBI agents constantly roaming around. Possible outcomes: Facebook quality drops significantly as they lose valuable staff; Facebook service gets disrupted and usage plummits as servers get taken at random, or entirely; Facebook gets a big, fat, "This Website has been serving Illegal Content" notices with the US seal on it.
3. Current admin uses social media to strongly promote the idea that Facebook is working against American interests. Pushes on Silicon Valley location, "presidential harassment," censorship of conservatives, etc. Starts a smear war. Facebook afraid to push back and thus "confirm" propaganda. Possible outcomes: Usage drops from a boycott (oh well?); radicalized militia/crazies start bombing / shooting up Facebook offices, Facebook forced to shut down US operations out of security concerns / staff flee en masse out of fear.
I'm not claiming any of these have even more than a 20% chance of happening, it's just fun for me to explore the possibilities.