Fox News gives the crazies a single, read-only feed with a limited amount of ad space and at least some regulatory oversight. The largest "congregation" you could make with that is to ask all your neighbors to go to your Fox News viewing party. Furthermore, other channels with different (opposite?) biases are a button press away. Biased media has always existed and didn't lead to this kind of large-scale polarization, so it seems like a read-only feed is mostly safe.

Facebook gives the crazies a town square and a set of megaphones, the volumes of which dynamically vary depending on who's at the receiving end to maximize "engagement" (which usually translates to outrage or arguments). The size of the congregation is virtually unlimited. There's also no "channel" you can just switch - if Facebook thinks you will "engage" with a certain kind of content it will slowly but surely start feeding you more and more of it, which would also draw in and radicalize people that were otherwise well-meaning. This is a whole other level compared to a biased TV channel, and something society hasn't dealt with before.