Earlier quoted context omitted.
I am wondering the same thing. It's interesting that they didn't report at all on the median and only the average. Also find the timing interesting, as I can't help but suspect that both the justification and incentive for self-reporting a higher number of friends materially changed for some people in the early days of social media. They didn't seem to acknowledge this at all. The model they built that draws a causal…
I think you left out the biggest one (though I suppose #3 indirectly hits on this). Social media, and increasingly even online media in general, tends to heavily misrepresent 'the other side.' In the past relationships were formed primarily in person so you actually got see what 'the other side' was like. Now a days people instead depend on completely inaccurate stereotypes that are far more like cartoon caricatures…
Polarization could instead be because there are fundamental differences in how people see the world and what is right. And now that we've tangled ourselves through all the wars imaginable to dispel the old division lines, this is what we're left with. This is what we have, now that information has become available for the masses; the real differences which split people. Not based on phony dividers of the past.
Polarization also means that if you disagree with the ideology of your family or of your village, you have millions of friends on a national or international level who think like you, instead of being ostracized for life.