Dude, I would not tolerate that whatsoever.
I broke up with my fiance of almost 3 years in September. I didn't pull that crap, and I can honestly say I loved her, and I loved her until the end. I am still financially broke because of that relationship, even though I got her a job and did so much more for her. A few days after the break up, I hit the ground running, because I'm stable and there is no use living in the past, although I occasionally experience short pangs of depression (they are less and less as I meet other girls) and a little anger. The point is, I got the heck over it, and it was a real throttle to my world, just as much if not more so than this girl experienced I bet.
This girl has some issues, and she is EMOTIONALLY cheating on you. She ought to sever such ties like MySpace, if she is mentally healthy, and make a new one. Saying she misses him is publicly insulting you, it's disgracing your honor publicly. I would not stand for such disrespect if a girl did that to me. I would honestly dump her ASAP, unless I slept with her (personal thing - religious obligation for me).
If this girl truly cared about you in any way, she would not have said she missed her ex, especially publicly. She honestly does not care about you, she is merely using you as some person to fill her void slightly. You need to man-up and get over it, and realize there is someone out there for you, and not take this. Suck it up and dump her, or in the very least, tell her that her behavior is unnacceptable and if she does not delete that MySpace and never pull such a stunt again, and get rid of other ties to the guy, you're dumping her ASAP. Don't be mean, don't be angry, don't be emotional. Just stand up for yourself and get the job done, and move on.