He talks to me on facebook but dosn't acknowledge me in person? help?

[Arixoxo]

Ive been speaking to this guy from my school on facebook for a couple of months now (im a girl btw) and I kinda like him. Hes really flirty with me on facebook and sometimes I flirt back. We play question games where we take turns to ask each other questions (on facebook) and he sometimes asks dirtyish questions and I asked him if he likes me and he said kind of. My sister and his sister are best friends (they are about 20) and they keep talking about me and him and keep saying we should get together. However, in school he walks past me and dosnt even look at me, he comes over to my group of friends and talks and looks at everyone apart from me and he just acts like im not even there. Do you think its embarressment or that he likes me alot or something? But he still talks to me every single night after school... I dont get it? Ps.I did say to him one time 'how come you didnt acknowledge me today when I was walking behind and you saw me aswell ' and he said 'oh sorry I didnt realise'