Why am I threatened by this girl? On Facebook, of all things!

[neltuneliel]

My boyfriend and started dating 6 months ago. But didn't really start a relationship until about 2.5 months ago. I didn't want to go official on Facebook because I thought it would jinx our relationship. But for some reason, he really wanted to. Once we did that, all hell broke loose. Girls on his page started going crazy saying to him "I want to be with you!", "I thought you liked me", "You are such a player!"...he told me all these things...but they didn't really bother me. It's this one girl, who constantly begs him to hang out with her. She's left messages on his phone, crying and begging him to come see her because she is "lonely". She's told him that he "should have hit that when he had the chance". I basically told him this girl is bad news. And he said he's never going to hang out with her. BUT she still asks him on Facebook, if he will hang out with her. He never tells her "no", but leads her on, making her think he's actually considering it. Last night she asked him if they could go out for ice cream. He said he might take her up on that...Why is he telling me something different to what he's telling her? Should I be worried?