How come shes so comfortable with other guys but not me?(10 points BA!)?

So i like this girl.. Long story.. I've talked to her on FB before and most of them lasts like 2 hours or so.. i've talked to her in school.. like i said Hi once and well she looked down and looked up at me for a second and said Hi and quickly looked away.. and in cl during a activity she seems scared to maintain eye contact and reluctant to talk to me.. but on FB shes friendly and seems happy to talk to me.. i can see her other friends who are guys she is so comfortable with.. it isn't that hard for her to initiate with them too.. but she told me once she never IMs first which is why she doesn't im me first.. but she is comfortable initiating with other guys.. she has NEVER ignored me either.. and i thought she showed signs she liked me(like actual stares before but their on and off). whats the deal with her? it seems easy for her to even initiate or post on other guy's walls but she always wants me to start with her..