Hmmm... I'd go with what Hopeful25 said. Let her know what's going on and see what happens... that'll tell you everything you need to know.
Why do you put up with it though? Well, quite honestly I'm betting it's because you don't have many friends and are desperate for attention from whoever will give it to you.
I know, I make that sound so horrible, but I also know I do the same thing myself, and I've gone through what you are right now. I didn't want to accept the fact I was just being used and that no one cared, so I ignored all the warning signs. It was a bad call though because I ended up being totally screwed in the end and ended up so much worse off because of it.
So yeah, see what she does when you confront her about it. If she gives you a bunch of **** then move on. A real friend will give a crap if they hurt you or not.
Now keep in mind some times we hurt the people we care about the most, so maybe that's what's going on here. For all you know she feels like other people are using her the same way you feel she is using you.
Also understand that complaining to someone and hanging out with this are two different things. I complain to a friend of mine ALL THE TIME. I absolutely lover her, but I also know I say things that hurt her some times and I don't always make an effort to hang out as much as she might want to (simply because of how messed up in the head I am).
Maybe she's the same way. *shrug*