Facebook is being accused of undervaluing the intellectual property it sold to an Irish subsidiary in 2010.
The company's subsidiaries then allegedly paid royalties for the intellectual property rights to a US-based parent incorporated in Delaware, where intellectual property is not subject to taxes.
Facebook stands by the decision according to Reuters.
Facebook spokeswoman Bertie Thomson told the agency the company made the decisions in 2010 when it "had no mobile advertising revenue, its international business was nascent and its digital advertising products were unproven".
Those digital advertising products and their ability to hone in on and aggressively target specific groups are well and truly proven now.
Ads on Facebook and its other apps brought in almost $110.3b in 2019, for a profit of almost $37.8b.
Facebook owns Instagram, WhatsApp, and Messenger in addition to the social media site.
Almost 3 billion people use at least one of them each month.
Only the company that owns Google makes more money selling ads on the internet.