it seems weird to say a mainstream social media platform cult following.

I agree the word choice is a bit odd. I meant in the sense that Twitter had a dedicated userbase that used it a lot and sort of glorified it. You weren't talking with your friends or posting online, you were "Tweeting at your mutuals". The most succinct way I could put it was "cult", in the same sense that movies and TV have "cult" followings: a dedicated group that enjoys and bases some part of their personality around it.