Yahoo told Facebook on Monday that it must pay licensing fees for 10 to 20 patents, or face a lawsuit. The patents cover systems for "advertising, the personalization of websites, social networking and messaging," according to the New York Times, which revealed the new patent fight last night.

Yahoo confirmed its position on the patents to the Times, saying “We must insist that Facebook either enter into a licensing agreement or we will be compelled to move forward unilaterally to protect our rights.” Facebook is still evaluating the claims. The specific patents at hand have not been revealed.

The once-mighty Yahoo attempted to buy Facebook long ago, back in 2006. Having never created a viable social network to complement its widely used e-mail and search services, Yahoo ultimately settled for integrating Facebook into its own sites. Last September, it was revealed that Yahoo is exploring various possibilities including selling off individual business units or even selling off the whole company.