It is obviously social media: more connections does not equally mean social interactions - people substituted costly and hard in-person for cheap and easy online interactions, with AI optimization fueling the polarization.
I get why social media could possibly be the cause but what makes you so certain it is the cause or the largest contributing factor?
When did algorithmic recommendations (by which I mean injecting content into your feed to maintain attention, rather than an attempt to rank by quality) become commonplace? ISTR Youtube was being criticised for pushing conspiracy theories in the late 2000s, but I could be misremembering the timeline.