I wonder if this girl is using me as a place holder for her ex?

Recently started talking to this girl, and Im not sure if she's using me until she can see her ex again or if she is really in to me. We text throughout the day, she stays with me on the weekends (we hook up every weekend). The problem is though, that her ex is in another state and she doesnt know that I know about him but its on her facebook about how she misses him and he misses her (I dont stalk, Im rarely on it) but Over christmas break from school she put his name in her status when they were hanging out. Im just not sure what she wants me for if she's still into this guy.