A feature of the internet has been the growth of networked zealotry; where intensely held attitudes are expressed in overheated rhetoric and ad hominem abuse, not as solitary aberrations (though that also happens), but in self-reinforcing internet coteries.
This has fed off, and possibly intensified, the bitter "culture wars" of the US; the intensification of political rhetoric and a more intense partisanship in politics. The last, particularly in Federal politics, has likely been fed by a change in living arrangements among Senators and Congresspersons whereby they spend much less time socialising with people of the other political Party.
There is some penchant--depending on political preferences--for blaming one side of politics or the culture wars more than the other. Since the phenomena in question occur across the political spectrum, this is not likely to be an analytical fruitful exercise, as distinct from another manifestation of the same patterns.
Drowning not waving
Looking at deeper structural changes, we are living in information-saturated societies. A natural response to such an assault of information is to retreat into simplifying and emotionally satisfying narratives. And what is more simplifying and emotionally satisfying than a Manichean story of good and evil, where you and those who think like you are the "good guys" and them over there are there are the bad guys, who clearly only believe what they do because they are evil, wicked, malicious and stupid (while you and yours believe as you do because you are moral, clever, smart and informed).
The dramatic drop in communication costs, and explosion in ability to connect, that the information technology revolution represents means that the like-minded can associate together far more easily. This can be liberating and reassuring. It can also lead to intensification of beliefs as people reinforce each other and divergent information is excluded or discredited. An ironic effect of massively increased access to information is to make the crippled epistemology (pdf) which is so much a part of zealotry and fanaticism easier to maintain.
The established information institutions have (more than) done their bit to create the basis for these patterns. While critical thinking is allegedly an ideal of post-Enlightenment education (particularly universities), what educations systems have generally actually been teaching and practising (particularly universities) has been groupthink. (Scott Sumner, for example, regularly bewails the current groupthink among macroeconomists--all the more remarkable since it fails to conform to previous accepted analysis.)
Moreover, teacher and academics may have been better at teaching the habits of groupthink than their specific groupthinking. Particularly if alienation from the offered groupthink leads, not to open-mindedness, but the search for more congenial groupthink. A sense of status, worthiness and morally-charged meaning are powerful passions; and if the real "lessons" have been that that is what information and analysis is "for", then people will go off and search for it. And, thanks to the information technology revolution, very easily find it.
[Read the rest at Skepticlawyer or at Critical Thinking Applied.]