There's legal recourse for this (see Chaplinsky v. New Hampshire, among the many other SCOTUS rulings curbing free speech in special cases). Social media companies are not the arbiter, the courts are.
Fighting words only apply to face-to-face speech and even then it's somewhat doubtful. If you want further analysis, look at some of Ken White's writings on it on Popehat. Incitement as defined by the Brandenburg test is more likely to apply.
I dont expect FB to be a arbiter, but in a more general sense, prevent harm done using the facility. Like a transport company to be held liable if someone is using them to mule drugs, or banks for money laundering.
