I hate FB as much as the next guy but I think that Facebook is an amplifier of other trends. I think that the underlying issue is the two party system. The echo chambers get amplified.
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.