Eminem's publisher is suing Facebook over its use of an Eminem song in an advertisement for Facebook Home. Remember, the elements of copyright infringement are (1) ownership of a valid copyright and (2) infringement without exception. Infringement is shown by proving a defendant had access to the copyrighted work, and the two works are substantially similar. More access means less substantial similarity is required. Less access means more substantial similarity is required.
Here, the article says the song is a slight variation of the Eminem song. This would mean that there is substantial similarity between the two works. The article also mentions there is an online trail proving that Facebook's founder was an Eminem fan. This would show access. Eminem's fame in general also goes to proving access. So, if Eminem can prove he has a valid copyright, he probably has a winning case.