I have two people blocked.
One is my toxic, horrible, Queen Bee co-worker. As a rule, I don't friend my current coworkers on FB (I'm friends with coworkers from my former job, and if I still worked there, I'd probably friend them, but it was a friendlier environment), but this one is someone that our lives overlap a lot outside of work, and I know we have many mutual friends. I want to have as little to do with her as possible, and blocking her makes my life minimally easier.
The other is someone who was offended when I told her she had to follow the rules of the group that I am an admin for. We are a support group tied together by a common medical issue, but we are very clear that we are not allowed to give medical advice and that anything that is posted that could be construed as medical advice will be deleted. She was convinced it was because she is into natural alternatives, but the owner of the group uses some natural alternatives and likes them, but she also doesn't want anything like that on this group. This poster got a little too aggressive with me, ended up banned from the group completely (with the agreement of the owner and all of the group admins) and I blocked her. It has been long enough that I could probably unblock her again.
I do unfollow some friends. For many, it is like others have said - politics! In one case, though, it is a friend who loves to post food pictures and make comments like 'NOM! NOM!' (I really dislike that phrase). I like her, in short doses, so I just don't follow her and when I know I'm going to see her, I'll pop onto her page, catch up on her posts quickly, and maybe even like one or two of the most recent ones (she also posts about 10 times a day), and she really doesn't seem to realize that I'm not following her.