That is a fair concern, and I share it, but one can hardly blame that on social media. The Romans didn't have anything like our modern conception of social media, and they managed to collapse just fine without it. The problem as I see it is that fighting other Romans became more profitable for the Roman decision makers than fighting their neighbours.

Social media is an effective tool which can be used as a weapon, but the problem isn't that Americans have effective weapons; it's that they're incentivised to use them against each other. The solution to me seems pretty obviously a return to federalism. It's the fact that there is only one throne that makes everyone fight for it with whatever is at hand rather than build things up locally.