Most Helpful Girl
I know you said you didn't tell her you liked her, but in my experience, a guy rarely messages a girl on Facebook unless he's family or he's interested in her. She probably picked up in the fact that you like her and decided to cut it off rather than give you false hope for something more than a dead end road.
Either that, or when you messaged her she saw your name, asked herself who you are and why she's friends with you, and unfriended you because she has no idea who you are.
Most Helpful Guy
You can't get an explanation of everything >.< Have you ever spoke to her in person? if not that shows you aren't confident buddy. Girls like confidence. Also your display picture on Facebook is what the first impression would be. If she didn't like your first impression she probably isn't into. You need to find another way to approach her or move on. Not all crushes can be dreams come true.