The US Internal Revenue Service is suing the world's biggest social media site over billions in taxes it alleges the company avoided for years through dodgy licensing deals.
The IRS is chasing $14.1 billion from Facebook.
The lawsuit comes after the "Double Irish, Dutch Sandwich" tax loophole beloved by many corporations was finally closed at the start of this year.
That loophole allowed Facebook and others to minimise their tax obligations by setting up shell companies in a variety of countries and moving money between them until it eventually disappears in the eyes of the taxman.
This is done by assigning the intellectual property of the main company to a subsidiary then paying it licensing fees.