Is a Friend a Friend on FB?

I know that this is silly, but it's just a thought. We have all done it. Accepted or made a friend request to or from someone that isn't really a friend. They aren't rotten. They aren't vile. They aren't your friend either.
Why am I asking such a silly question? Well, I recently discovered that I was unfriended ((GASP)) by someone that I thought was my friend, and not just on FB. It was surprising (we've been friends for a few years). I just saw this person and spoke with her the other day. Nothing was awkward, and certainly nothing happened for me to wonder if we were "friends."
Today I just happened to go to her page and realized we weren't friends anymore.
Now granted, this is not the first or last time I will be a casulty of unfriending. A few dirtbags have dropped me. Kidding.:)
But, I also know that the only people I've dropped are people that I don't desire connection with anymore (and maybe never did, but hit accept without really evaluating). Was just wondering what you think of FB friends that unfriend you that you considered a friend.
Do you ask them about it?
Do you let it go?