What does she say when you ask her about this?
Yes, social anxiety and depression certainly could be contributing to her behavior, but "anti-social" sounds harsh. I wonder if she is uncomfortable about leaving the house, or if something is going on at school that she is avoiding, or if she is so depressed that she just isn't interested in anything, but only she can tell you these things.
Is she already receiving treatment for the social anxiety and depression? This seems like something to discuss with her therapist?
Try inviting her for any outing with you that you are fairly certain she would enjoy. Insist that she go, for your sake because you miss her company, even if she doesn't want to. See if you can have a good time. If she is absolutely resistant to going, or if she is not able to enjoy herself when she goes, then I would definitely seek professional help if you haven't already.
Ideally, if you know what the problem is, you can better know how to help her. Being an introvert is one thing, but never wanting to see another human being is another.