It's kind of hard to argue that Facebook has a monopoly on "social media" when there's so much actual competition for attention in the same space. Twitter, TikTok, Snapchat, iMessage, Youtube, etc. Yeah, there is no other Facebook-style social network to share photos of your grandkids or whatever, but no one under the age of 30 even uses that these days and it's hard to argue that it's as deserving of government intervention as oil, railroads, or telecom in general were.
I'd be looking much more closely at Amazon (nearing a monopoly on e-commerce, soon to be all commerce) and Google (an actual monopoly on search) if I were the government...