Can I ask, hopefully without offending, how much effort you put into making potentials into actual friends?
If nobody comments on your Facebook status- do you regularly comment on theirs? Like pictures, reply to comments, etc?
If you don't get invited on events- how often do you invite them? Do you make it known that you fancy going out? Do you suggest it?
How much time do you put into maintaining and nurturing friendships? Do you regularly make the first effort for contact? Suggest meet-ups? Send them a little card or pressie in the post, or bring them in a bun, because you were thinking of them?
I was guilty of being quite a loner. I'm single, not easy to find a babysitter, and can quite easily sit on my lunch at work and read a book, rather than chat. It's only when I've actually prioritised being a friend, that friendships have become real. I started inviting a friend round to my house for dinner, since I couldn't afford a babysitter. A newly single colleague was lonely, so I've made the effort to message regularly, ask how she is, take a picture of something that would make her smile. She's now a friend. A close friend at that.
All that energy was absolutely exhausting, to be honest, as I'm very much an introvert. But it's worth it when something happens, and I have friends rally round, or when I do get tagged in FB statuses and I feel included.