I only ever unfriend people who I have absolutely nothing to do with anymore, haven't seen for a couple of years, and who have not acknowledged any of my posts in a year or more. As a pp said, if they are not in my life anymore, I never see them, and I have no communication with them (or them with me,) why are they on my friends list? 
I have just 65 friends on Facebook, and they are all people I know, have contact with, see, and speak to. I see some people with 1500+ 'friends.' Makes me LOL, No-one has that many friends. One person I know who has 1350 'friends,' had a big birthday last week (40,) and they had birthday greetings from 53 people. Out of 1350. So 96% of the people on their friends list didn't acknowledge their birthday. Proving (IMO) that the vast majority of them are not actually friends. Some probably don't even actually know this person!
I mean, I wouldn't be Facebook friends with my friends husbands anyway, and do find that a bit odd. Unless I categorically know him myself, and we are friends, I would never be friends on Facebook with a friend's husband. If we were friends, (in real life,) and he unfriended me on Facebook, I'm not gonna lie, I would be puzzled. Probably would ask my friend if I had offended him because he's unfriended me.
As a pp said, it sends a very clear message when someone unfriends you... 'I don't want to know you anymore, and I no longer want anything to do with you.' It is quite a rude thing to do IMO, and if it was someone I was in fairly regular contact with/got on with OK, I would be puzzled and would possibly message them to ask if everything is OK.
Not being funny, but no-one has ever unfriended me on Facebook. I have unfriended people though. But as I said, they're people who I have not spoken to or interacted with for a year or more (and didn't have that much to do with anyway.)
.