The premise of this argument is false. In Germany, where I live, polarization has increased dramatically, especially since COVID. I don't know about Facebook, but the tone on Twitter is harsh, the hate is palpable. The difference is maybe that unlike in the States it's not a 50:50 split, but maybe 80:20.
NBER's study [0] found (West) Germany had the biggest decline in political polarisation over the 1980-2020 period of all the countries they included in their study (12 OECD countries).
Doesn't entirely contradict your position, given that it was specifically measuring polarisation in terms of attitudes towards political parties, and so may not be good at measuring forms of polarisation that do not map straightforwardly to political parties; and looking at long-term trends over 40 years doesn't tell us much about how people have reacted to something which has only happened in the last 18-24 months.
The study failed to find any statistically significant correlation between political polarisation and proxies of social media use (Internet penetration and online news consumption; the study authors did not have data on social media use itself)
I'd also like to see references on which these countries with declining polarization and high FB/IG-usage would be. It's not a trivial or uncontroversial thing to quantify but hey, MZ brought it up.
