The dissolution of a relationship is always sad. You have to confront the fact you are no longer on good terms with another person. You think things are ok, but are never sure. And then, you see it: You've been unfriended on Facebook. It's now official.
There's that little pang that hits you in your core. You begin to think, did I do or say something to elicit this?
Quite often, it's not your fault, but theirs. They wanted to end it before you did (or if you ever did, at all).
But why would you want to continue being 'friends' with this person online? Part of me believes it's the last lifeline you have to them. There's still a chance for you to outreach to them, and vice versa (even if they're not interested in being the first to contact you). And having that knowledge, that thread of communication, keeps you from losing your mind.
True, going through Facebook is a passive-aggressive way of doing things. It's just as bad as sending a text instead of talking to them face-to-face (back in the day, it was done with an actual phone that people spoke into instead of typed with). You should get a proper answer, out of respect, even if it makes little sense to you. Unfortunately, not everyone is respectful.
So, even though Facebook is a relationship ender, it's not the be all and end all, communication-wise. There's still email, and the telephone, and perhaps a carrier pidgeon for traditionalists.