Girl acting strange, Read Description?

[Robert]

Hey all,
\im friends with this girl, i go round to hers and we flirt, talk and have fun together, We have even done sexual stuff (apart from sex)
This girl hugs me, and makes me feel loved. We are both 15, However wheni try to talk to her via facebook, i get ignored completley, she talks to other boys sometimes. I dont even annoy her, i just say "Hi" and i dont get an answer, like ever... This girl is very confusing, can anyone help as to why she ignores me via facebook chat, but in person we are perfect together. I sometimes thinks she is using me, but im not sure.
Can anyone help as to why this is?

Any help Appreciated

Thank You