I agree with Marcie B's suggestion about signing in for her.
However, at 11 shouldn't she be spending more time with real life friends and doing other things other than myspace? I'm not saying that's all she does - I don't know your family at all to make that judgement. But if she's on there enough to be changing passwords and telling boy's she'll make out with people (AT 11), she's on there too much.
Kids being on the computer has advantages as well as disadvantages. They need to learn the technology - HOWEVER, they need to be kids too. At 11, she is fresh out of elementary school. Granted she'll be hitting puberty soon, but she shouldn't be growing up faster than her age. At 11, holding hands with a boy was enough for me. I spent time with my girlfriends' houses, having sleepovers, and all that.
Nowadays, kids spend more time on the computer than they do in sunlight. If you don't put clear and firm boundaries and follow through with making her pay the consequences, she'll take the reigns right out of your hands and do whatever she pleases.
I'm sorry I'm rambling more than advising. I hope you find the right answer to your situation. Good luck.