If FB were neutral they would show you every FB post, millions per second whizzing past your screen, but they can't do this, they have to curate a wall for you to slowly scroll through and for most revenue, like, share, or comment on.

Therefore, to show you the most content that you will like, share, or comment on, they repeat the type ($x) you've already liked, creating the echo.

So no, it is not mostly a problem of the underlying issue of the two parties, this is entirely about how FB curates your wall and simply doesn't show you "the other party"/$y or anything deviant/$y of your likes.

Edit: changed political parties to variables to illustrate point.

30 years ago an R and a D could cut a deal to get things done and few people would notice that they compromised by giving a little to get a little.

Now when such deals happen the deal makers are branded as traitors and RINOs (do people use DINOs too?) and must be primaried.

FB encourages polarization because it increases engagement with their advertisers, which is useful to FB. The polarized base is useful to parties because it motivates them to donate, proselytize, and vote. That base polarization leads to polarization in candidates, and the division grows.

According to the article, FB is not taking a passive role in this; they're actively trying to exploit people.