Au contraire, there's a lot you can do within the constitution. Anti-trust law is one thing. Laws that regulate the behaviour of publishers (which is all Facebook is) are another.
In the last 30 or so years, the USA has completely dropped the ball on anti-trust. Compared with Europe, US citizens are paying more, for less choice and poorer services in, say, broadband and mobile services, because it's allowed local monopolies to develop. And the same thing has happened online: Facebook and Google have conned politicians into seeing them as like national flagbearers or champions against supposed European or Asian competition, which means they get supported rather than slapped down.