on mysace, if you cant find her by email, name, or display name, try looking for a good friend of hers. If you an find out her best friends name or a boyfriend look for their name. just type in their name, then set the area code to where they live and lower the miles distance from area code. Once the friend is found look at their friends listings till you find the girl your looking for. But I will tell you that if shes switching emails to try and keep it hiddin, she will have the profile set to private so you cant look at it. So what I would do is create a fake profile and request to be one of her friends. Once she approves you can look at her profile.
For facebook, you can only use your email address your school gave you, they only give you one. So the one she has should be her normal one look on there for her friends. Copy down their email address (Facebook does provide those openly) take all those emails and search on myspace with them.
Hope this helps.