I'd argue that fear not Facebook brings out the worse in people - fear about income or place in society.
In my view, the real cause of the recent rise tribalism is rising social inequality and corrupt politics doing nothing about it.
You could argue then why hasn't democracy acted?
I'd argue that it's paid for content - whether it's ads or partisan coverage, lobbyists, paid shills, or astroturfed pressure groups all managed by professional manipulators ( ad people ) - that's the problem, not the medium ( Facebook or TV/radio or whatever).
Why haven't politicians acted decisively on climate change? I don't think it's the Internets fault.