I think the timing weighs against the idea that the Internet is to blame. Pew have asked US adults whether they use the Internet, and most of the change in Internet use happened in the 1995-2004 period (14% to 63%), not the 2004-2014 period (63% to 87%). One could tweak the Internet-polarized-people hypothesis so it better fits the timing, perhaps by invoking a decade-long time lag between Internet penetration and political effects, or by substituting something like "social networking" for the "Internet". But I would want to see an argument to back that up.

Online shopping or wikipedia isn't going to polarise people. I'm sure many people here were early adopters, and hung out on usenet or mailing lists, but this was not the norm. It was around 2005 when myspace turned online socialising into something mainstream, and 2008 when not being on Facebook was actively contrarian, and a few years later when even the contraians gave in.

Furthermore, in the earlier days blogs could express complex opinions. It was only with facebook and twitter that opinions boiled down to one sentence.