Why is he ignoring me?

There was a guy i met a couple of times a while back and on the first occasion we got on really well and had a deep conversation and he talked only to me and paid me very big complements. We had alot of things in common. Even people around us thought that there was a connection because a friend of mine moved so we could sit next to each other and talk more. (we were in a restaurant). A friend of mine observed that he was looking at me alot when we were't speaking and i thought this too. After that day i added him on facebook and for a week we talked alot and had really deep conversations about big things like life, religion and sprituality and he complemented me alot again, the for some reason he began not talking to me after this week and totally ignoring my messages on facebook, at first he gave the occasional hello but now hes completely ignored me and i knows hes been on facebook. ( i will admit that i have always had to initiated the conversations online but he's always been into it afterwards).
People say that i am a sweet and lovable girl and there are some people i know who think i am beautiful looking, i have been called pretty by some people aswell. I am quite insecure about myself and i have never had a boyfriend before (im 19 by the way). I can be very shy and introverted but can liven up sometimes too.