Many of them are literally bots. Many have the same type of responses as the bots on twitter do.

Most of the major fakenews is upvotes by bots at at least a rate of 9 bots per real person if not far more, especially for the propaganda subs that "emerge" every so often with 1 single post and then are never seen again.

The only place it's "worth" arguing is subs that have developed their own culture and massive set of in-group jokes, as the bots stand out more there, get downvoted more for not fitting in, and there are more lurkers reading because the content and comments tend to be more amusing. So basically Political Compass Memes? and no other subs at the moment, and I fully expect that sub to be banned soon.