I like Venkatesh Rao's perspective on this, in his blog post about "escaped realities". He argues that our games and mental models of the world are becoming closer to reality over time, "less escaped", rather than more escapist. He points out that geography is a much bigger filter than online groups regarding the variety of ideas that a person is exposed to. If anything, the internet, overall, seems to be widening people's perspectives. (Although it also lets people get better organized with their tribalism.)

I think the United States is undergoing a sort of cultural civil war right now, which I think makes the perspective of people living there somewhat different than people living in other parts of the world. I don't think that it would be as easy to make the case for "the internet encourages tribalism in politics" in other countries. I don't think that the internet is the reason for the social tensions in the United States right now, (and there are many.)