If Trump moves to start an anti-Trust suit that is completely justified by their actions, it will serve to rally their base and can be used as justification for all sorts of major or minor riots led by their AntiFa thugs.
If he doesn't act until after the mid-term elections (and I believe he will start an anti-Trust action sooner or later) they shut down a large portion of the information and commentary flow of their opponents from now until the mid-terms which they believe will help them hold on to and gain seats in Congress.
And the problem is (for social media), they are already operating as monopolies for the most part.
As an aside, their customer base is not just the Left. If everyone outside the Left starts giving social media the NFL treatment, I think they will notice. In fact, that opens them up to charges of financial malfeasance if their stock suffers because of their political leanings.
I would hope no action or inaction would be based on any concern about this. If law enforcement can't or won't put a stop to it we do have National Guard. President Trump needs to make decisions based on their merit, not whether or not there may be protests or violence as a result.