When it comes to rejecting friends, I think of it in terms of a two sided scale. What/how much have I given them? What/how much have they given me? How does it compare? Are the two so out of wack, or closely matched?
In my case it came down to realizing that I was giving way more than I was receiving. I had a bunch of friends on FB from high school, who never kept in touch with me, or who brushed me off when I tried to chat online, or never replied to messages of "Hey, what's up?" And I remembered, they weren't all that nice to me in high school. We were only FB friends because of nostalgia, for the sake of memory, so I shut them out, and erase them.
Another factor is where was this friend when I needed them? A friend of mine was going through a period of severe depression, due to job, school and girlfriend woes. I'd call to check on him pretty frequently, and we'd talk for an hour, sometimes two. I'd try to support him, give him advice, buoy his spirits. Last I heard, he seemed to be doing much better.
Yet when the table was turned, when I had been laid off, when I was so depressed that I wound up in six months of therapy, where was he? He never called me. And the final straw was when I was in his city (we live four hours apart) visiting family, and, having a free afternoon, called him to hang out or hit some art museums. He declined saying (no lie) he "didn't feel like going out." When I needed a friend most, he could get off his goddamn ass.
So I deleted his number from my cell, and unfriended him. I haven't spoken to him since then. He recently tried to refriend me, apologizing for not having kept in touch, and I declined the request. What he did to me after what I did for him is simply too much for me to forgive, and honestly if he were to spiral back into depression and snuff himself tomorrow, I wouldn't care in the slightest. He deserves to be alone. I now choose to devote my energies to those friends I have remaining, to not repeat his mistake, and to make sure they know how much I value them!