I will say one more thing on the matter. In college, any woman that's halfway attractive or social is going to be surrounded by men as part of her everyday social/interactive group, be it in class, after class, going out, whatever. You may assume that 50% of these guys have romantic designs on her in one way or another. That's generally the way it works.
You can't assume they're all douchebags bent on taking her away from you. You can't assume they post photos on Facebook to mark their territory. If you assume that, it makes YOU look like a douchebag. They aren't your problem, man. They're just in her sphere of influence. Cut them out of your picture. Focus on her. Accept that they're there, but just do your thing. Even if one is her ex, if he's still in her life, as YOU said yourself, that's her problem.
Anger isn't attractive to women. Lashing out at guys you don't know just makes you look possessive and crazy, which will send most of them running for the hills.