A.H.
First, you have to be at least 14 to open a MySpace account. (So, you already know your daughter lied about her age if she has an account in her name.) What else is she talking about? Where are the adults in her life to protect her? As a parent, you should have access to her account at any time. With that being said, you can only monitor her when she is with you considering her father is not responsible enough to handle his parential duties as anywhere near important.
Some parents make their kids give them the password to everything and if the child refuses or changes it w/o telling them, then the child isn't allowed to use the computer. MySpace and other online sites are down-right dangerous these days and for parents to turn their backs and think that playing on the internet is as safe as playing in their own backyards are parents playing parent with their eyes wide shut. You need to find a way to open the lines of communication with your daughter and not try to be her friend. You may find resistance at first, especially if she wasn't raised from a young age with the open communication, or her friends are telling her otherwise. Many parents opt to be friends with their kids and forget to parent them. You must be a parent first and foremost, and the friendship will grow from there as the child grows, matures, and enters the world. Parenting always should come first though.
As for her father, he's another story. He sounds a lot like my ex-husband and father of my two younger children. I got custody of the kids full-time though, and after regular visits in he past, he now comes around once in a blue moon, since he got remarried. He has another child (a boy) with his new wife as well as a son from her first marriage living with them.
You can't change anyone but you, so change how you parent and the rest will fall into place. Don't take a second seat to your child. Be in the driver's seat and be an active parent. You may want to check out some parenting books from your local public library. (I'm sure your reference librarian can recommend a few.) Also check in your community for support groups. Start a parenting group in your area. I'm sure there will be other mothers that are willing to come together to share ideas once a week.
Good luck!