Yes, I think social media is the main driver of all the negative changes. Social media is the opposite of a forum in lots of ways, because it is always about virality, the outwards ripple of attention.
When MNHQ started chasing social media engagement you could tell how it instantly started to corrode the community bonds here. That's why mediocre threads like Penisbeaker were destructive (though it was far from being the first thread with this effect).
And then of course social media started more and more to use polarising algorithms to assist virality, and platforms like Twitter became war zones. That nastiness crept onto MN too.
And it has crept into our real lives as well (Trump, Brexit, gender wars, etc) meaning that so many people are permanently furious and exhausted, ready to take out their frustrations on random posters.
The nastiness really has gone off the scale. There always was a mean streak, of course. MN used to be like the cloakroom's in an aspirational all-girls' school, patrolled by the elite gang who made caustic (but witty) remarks to pretenders. But now it is like the cloakroom of a failing inner city comp where everyone roams around punching everyone else in the face.