It becomes a problem when you've got people who are incredibly single-minded about something.
I don't mind my coworker's weekly posts about car stuff at all, even though I have no interest whatsoever. And when he posts some progress on some car repair he's doing, I'm glad to see he's doing something he loves.
But when my SJW friend posts yet another ridiculous over-the-top post about how all X are X, and somehow thinks this means they aren't be prejudiced themselves, it's hard to handle. You can't tell them anything, either, because there's something different about you that makes them think you're unable to empathize with them, or understand what they're going through.
Some of them are so incredibly obviously wrong, too. Like: All cops are corrupt.
This is a person who is pretty fun to be around and hosts parties that are fun. In person, they aren't so overbearing about this stuff. But online? They're a monster who can't be corrected or even debated with. If you do, you're X and can't understand.
In short, I think you're fortunate not to have anyone single-minded like this on your Facebook friends list. The only real solution is to just unfollow them, which is horrible because you then miss the posts from them that you actually want to see.