They probably have good discovery and trending post mechanisms. Mastodon makes it a point not to have one, which results in a wholly uninteresting feed for the average user. I'm only on mastodon, but I very rarely use it, because it mostly sucks unless you spend several, several hours trying to track down fun accounts to follow yourself.
That is my experience too. It's because there is no distinction between posts and comments, I think. It's about as interesting as my sms chats with friends would be to strangers.