I don't think it's very mature and appropriate of the mom to constantly bring up negative things to say about her son's girlfriend. If you guys like each other and make each other happy, that should be more than enough for her. You do allow your mom more say than she needs to, and this facebook stuff is stupid as you say. And I would be pissed to if I was your girlfriend as well. You guys will find your way and if you guys are unhappy, you'll break up and deal with it on your own. And dealing with it on your own is the best learning experience I can possibly think of.
Don't let your mom into as much of your personal life. I know you have to maintain a relationship with her, but if she starts with the bashing, I would remind her that it isn't her place and that you take offense to her comments. If she doesn't get the picture, back away from her, don't call your mom as much etc. etc. Focus on your girlfriend, the one that makes you happy, and make her feel as loved as you can. Focusing your energy towards a positive result can't really hurt things any more than they already have been.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.