Am I a friend of convenience?

I'm in my 3rd year of uni and I became really close with a girl on my course in 2nd year after my friendship group fell and everyone went their separate ways. We visited a few places over the summer, went on nights out ,attended events and had loads of fun together. I thought I had found a great friend but now I'm starting to wonder if I'm just a 'reserve' and a friend of convenience.

Since we have been back at uni I've noticed she only comes to me when she wants something or when she doesn't have anybody. or example we will spend time together before a lecture, walk into a lecture together and before I know it she has disappeared. At times we will walk into a class together and sit with a group of people, when the class is over she will get up and leave with the group whilst I'm still packing away for instance. When I do something for her she will not say "thank you" or acknowledge I've done her a favour. She does try to be everybody's friend, knows everybody in the year and bounces between friendship groups. In the past I have avoided people like that because it's difficult to develop deep friendships with them.

I know it sounds really trivial but I feel like I'm being used. Nobody likes being used, I certainly don't. What would you do if you were in this situation?.

Thoughts welcome.
Serious answers only please