It's no surprise that, since the Internet allows us to form communities based on common interests rather than geographic proximity, these communities end up self-selecting. Not just because the express purpose of the community's creation starts it off with a certain commonality (for instance, a forum for users of a particular technology not only select for people who like that technology, but also for the kind of mindset, education level, income bracket, etc. that would have that interest in the first place), but also because, once a community gets going in a particular direction, it makes itself welcoming to people who fit it, and unwelcoming to those who don't, and thus progressively reinforces itself. Even a small and possibly accidental perturbation in a particular direction can become a mission statement in time. For instance, a forum about restoring classic cars might not start out with a firm leaning towards one side or the other of the question of whether to restore to better-than-new condition, or to preserve rust and signs of age. But once a small leaning emerges because of a few outspoken or highly-placed people, this can sometimes cause a gradual shift, as advocates of the other side feel excluded and leave, while advocates of the preferred side aggregate more of their friends.
As a result, any given Internet user might find himself in a dozen different online communities with a variety of different topics of interest, and still can happen to find himself almost always amongst people who think similar to him about many things, and have similar overall outlooks and backgrounds, without even realizing it. For instance, I am active in communities about various kinds of technologies, about roleplaying games, science fiction, and a number of other hobbies. Some are fairly disparate: there's not much obvious overlap between classic arcade game buyers and restorers, and fans of David Brin. And yet there is, inasmuch as members of both groups tend to be at least somewhat well-educated, of an income level that allows enough disposable income for such hobbies, and enough free time to pursue them.
This, perhaps surprisingly, excludes a far bigger swath of my countrymates than might be obvious just listing those attributes. What really drives this home far more than any speculation or cogitation can is when you find yourself in a community that does not select on these sorts of things.
Most of the communities I participate in are full of what seem to me to be poorly-thought-out, irrational, vapid, and even stupid comments. One feels as if one is soaking in dumb sometimes when one goes out onto the Internet. And yet, a little exposure to a group that doesn't share those commonalities will make the others seem positively brilliant by comparison.
One particular community I participate in keeps driving this point home to me. Nearly every post made in it is so insipid that it reminds me of the sorts of things that, five years ago, you got forwarded to your inbox by your grandmother, with ten screenfuls of FW: lists above it. And yet this is the stuff these people sincerely feel and are actually writing themselves. Sometimes I think of quitting (I even did once but came back) just because of how completely worthless 99% of the posts are. Everyone is so completely credulous, and so suffused with saccharine cheer and optimism, and so utterly convinced that anything that was good for them must perforce be good for everyone all the time. They're the kind of people that, when stand-up comedians make fun of their trite normality, you feel like they're exaggerating and no one can be like that, but they really are. They're good-hearted but you can't help realize that they can go days, weeks, even months at a time between times they have a thought that wasn't spoon-fed to them, or seriously questioned anything.
I don't want to say what group this is, for fear of offending. Suffice to say that what brings me into this community has almost nothing to do with my hobbies, education level, background, income bracket, or any of the usual things that contribute to bringing me to a community. The only factor is a particular action in my past, which, though voluntary and also associated with proximate causes correlated with certain cultural and societal factors, is not really strongly linked with the sorts of things that shape the way a person thinks.
But after a while reading their attempts to answer genuine questions, even the comments on YouTube videos start to seem erudite. Those people might be rude, illiterate jerks, but at least their minds are able to question things.