YANBU to be hurt OP. It is a virtual slap in the face. 'I am no longer your friend and you mean nothing to me.' How can anyone not be at least a little bit hurt by that?
I have not 'unfriended' anyone for about 5 years, and even then it was only 'farmville buddies' who I didn't really know. I was unfriended by a mum of one of my daughter's friends a few years back though, after the 2 girls fell out.
I cannot remember how I noticed it. But I spotted the 'add friend' button which meant she had unfriended me. And there were posts that were available for anyone to see that said 'you do find out who your friends are don't you? Sick of backstabbing bitches' and some more shit that I won't repeat. Then you got the predictable 'U OK hun?' comments, and her saying 'can't say on here, but suffice to say I am incredibly fucked off with certain people right now.'
I had no idea what was going on, but found out later that her daughter and my daughter had fallen out, and apparently I had called her daughter a bully and a little cunt. (All lies. I didn't even know she and my daughter had fallen out.)
But yeah, that was upsetting as we got on well, and our girls had been quite close friends for several years, and there she was on facebook, unfriending me, berating me, and turning people against me. (Later on, she told the people in person that it was me and my daughter she was on about.)
Bitch. So glad I don't have that shit anymore. Now the kids are virtually grown.
I have had several others unfriend me, but like with the farmville friend, I didn't have anything much to do with them anyway.
But yeah OP, YANBU to be hurt. Don't dwell on it though. If you have little to do with them; stuff 'em. Move on. 