if you have no common interests and only saw each other after class for a couple of months than I wouldn't even call that a friendship. that is aquaintences. that doesn't mean you have to defriend her on social media. lord knows, I would have no friends left on facebook if I defriended everybody I didn't have a deep and meaningful relationship with...lol...nearly everybody is casual. but that doesn't mean you have to hang out with her. people may post on my wall or poke me.....sometimes I send a smily icon back just to acknowledge them, or a thanks or a like, but not always. if you get invited somewhere, you just respectfully decline.