I have this too, except this happened to me a lot during high school. I was good friends with this girl, then one day, she just stopped talking to me. Seriously, I sat beside her in class and then one day, I try talking to her and she completely snubbed me. The same happened to another girl I used to be friends with.
At some places I have worked, I thought I got on really well with the people there and added them on facebook after I quit. I no longer heard from them even though I sent a message or text. Nothing. But they keep in contact with each other though. Seriously I have no idea why people don't want to keep in contact with me.
All I know is, it's horrible to be abandoned.