At one point, Eminem was the most "liked" person on Facebook before his 'Love the Way You Lie' duet partner Rihanna stole the crown. One person he doesn't like right now? Facebook founder Mark Zuckerberg -- how cyclical, right?

Em has launched a lawsuit against Facebook for wrongfully using music that sounds "substantially similar" to his song 'Under the Influence,' from 2000's 'The Marshall Mathers LP,' in a Facebook ad.

Here's the gist of this legal mess.

Em's lawsuit claims that advertising agency W+K used the music as a way to curry favor with the social media site by catering to its founder's personal taste. It cites an example of the Zuck's childhood web page, which indicted he was all about the rapper.

The suit alleges that Facebook later altered the ad and changed the music, which the suit also deems an admission of guilt and proves that Facebook was aware that it had infringed.

Still, changing the music doesn't remove fault or veto copyright infringement, since Em's Eight Mile Style publishing company has ownership of derivative Eminem tracks, too. Eight Mile Style is seeking the maximum damages available under the law, which is as much as $150,000 per infringement.

The complaint also says that defense attorneys claimed that the song 'Under the Influence' was stolen from Michael Jackson.

What a mess. Whatever the case, it's up to the courts and legal eagles to deal with.

Below is the said-to-be-altered ad.

Watch Allegedly Altered Facebook Ad