OP, it would be wise to not post her Facebook here. It would cause a lot of other harassment issues and she would eventually change all of her privacy settings. Report your loss to the police, and did you even try to message her? Nicely ask that she found your iPhone and if she could return it at a public location, or offer to ship it at your expense. If she refuses, tell her that you have proof and timelines of the events, and remember to save her profile page and friends list as an HTML file in Firefox.
Edit: Search her last name in her friends list and find her parents, if she does not cooperate, find their phone numbers on WhitePages, and explain the situation and how their daughter found an iPhone that wasn't hers.