Why does my 'crush (guy)' log off fb as soon as i'm on it?

[Sweet]

we used to talk quite a lot over fb a couple a weeks ago (during half term he always started the convos.) and at college beofore half term. he always gave me signs that he liked me when we were at college.
but when we returned to school he wasn't the same as before. he talked less to me etc.. like may be once or twice.
anyway,
last week he quit college. i'm not going to see him any more because we never actually go to the point hanging around together. coz i hadn't talked to him for a long time i decided to start a convo. on fb. the other (online chat). i told him that it has been a long time that we talked, and how he was, he answered that he was ok but he had to go, he apologised and said that we would talk soon.

since then he hasn;t been on fb until today. when i logged onto fb he was offline but then swithced to online maybe 5 -10 minutes later he proper logged off fb. then an hour or two later when i logged back on he was on it again. now as usual he logged back off from fb after 5- 15 minutes.

but why is he actually doing this?? is he shy to talk to me and trying to avoid me or is it completely the opposite.

thank you and sorry for the length.