[ianai]

I think it’s more of a selection bias. The people most likely to comment online are the disgruntled people. I.e. in order for someone to take the time to comment or contribute to an online discussion or forum that person needs to surpass a certain amount of “care”. That desire to comment or contribute is much greater in aggrieved people than happy people. It’s a fundamental characteristic of psyche.

So I think social media does two things. It gives people a very small barrier to post. But it also combines them with other, like minded people. Those like minded people then ruminate on their shared aggregations and wind up more outraged than they would have been without that rumination.

[joe_the_user]

Well, one thing is the mere act of commenting on X tends to make one more committed to X. The process of becoming what you say is accelerated by people posting links, memes, cut-and-paste-me's and so-forth.

A person may not fully believe meme X but they may "find it interesting", may want to "rattle people's chains a bit". Then someone else launches a full attack on meme X and the person feels, attacked, and defends, attacks the other person and so-forth. Polarization is powerful and well-documented dynamic on the net.