Facebook said it might have to pay the feds between $3 billion and $5 billion
The IRS has been investigating Facebook for its 2008 through 2013 tax years. On July 27, it sent Facebook a statutory notice of deficiency related to whether or not it understated assets by "billions of dollars" when it transferred part of its business to an Irish subsidiary in 2010.
Facebook said that it plans to file a petition to the US Tax Court challenging the notice.
Here's Facebook's statement in the filing (emphasis added):
"While the Notice applies only to the 2010 tax year, the IRS states that it will also apply its position for tax years subsequent to 2010, which, if the IRS prevails in its position, could result in an additional federal tax liability of an estimated aggregate amount of approximately $3.0 - $5.0 billion, plus interest and any penalties asserted. We do not agree with the position of the IRS and will file a petition in the United States Tax Court challenging the Notice. If the IRS prevails in the assessment of additional tax due based on its position, the assessed tax, interest and penalties, if any, could have a material adverse impact on our financial position, results of operations or cash flows."