I think it's actually mostly a product of the overton window kind of being split, but, not really split in the ways that you might think. It's not so much that each party is getting more extreme in what their actual beliefs are, it's just that people are getting more extreme in their rhetoric, more extreme in their isolation. This, if anything, encourages people they disagree with not to vote. You need only look beneath these kinds of posts to see the sheer amount of people that chirp up, disagree at any time, and then totally fail to be convinced.

No, much like underpantsweevil said, this is purely something that's for the in-group, to make them feel good, to make them feel like they're doing something productive, and more than any of that, it's a way for them to work out ideological insecurities and reinforce pre-existing beliefs. Which further feeds into that rhetorical isolation. Keep that running for 30 years online (maybe more like 10 if you're just considering web 2.0), and you shake out to about where we are now. Continue this for another 10 or 20 years and you're probably directly headed for some absolutely heinous shit as the gulf gets wider and wider.