Does anyone else get this feeling?

Nobody's my friend. Every time I log into Facebook and I see pics of people having a good time with their friends, girls looking beautiful and nice guys with their arms around their shoulders, I think to myself, "I'm not welcome with them." The people in those photos do not hold me in the same regard as they do for others around them. Even if they show me kindness, it's really only at a superficial level. They oftentimes don't answer my messages, and I usually assume it's because they're not interested in talking to me. Every time I see a girl I find attractive, I think to myself, "She's not going to like me." I do my best to be sociable - I say "hi" in the halls, I have conversations about subjects of mutual interest (usually), I hang out with people, etc. But I still get this sense that I'm not really "one of them." I feel as if I don't matter. I'm just a presence. I'm just "there."