Yahoo sued Facebook on Monday, accusing the social media giant of infringing 10 patents.
According to the lawsuit, Facebook copied Yahoo's products, including in advertising, privacy controls, social networking and messaging.
The company said it made innovations in social networking with products like Yahoo Groups, Yahoo 360, Yahoo Mash and Yahoo Pulse.
The lawsuit alleges that Facebook infringed a Yahoo patent with its News Feed feature, a running stream of friends' activities. The lawsuit also claims that Facebook copied Yahoo's advertising features and its privacy controls.
In a statement, Facebook called the suit "puzzling."
"We're disappointed that Yahoo, a longtime business partner of Facebook and a company that has substantially benefited from its association with Facebook, has decided to resort to litigation," a Facebook spokesman said. "Once again, we learned of Yahoo's decision simultaneously with the media. We will defend ourselves vigorously against these puzzling actions."
The suit asks the court to order Facebook to pay triple the damages it allegedly caused to Yahoo "in view of the willful and deliberate nature of the infringement."
Although legal battles over smartphone technologies have become common in recent years, the suit marks one of the first major legal actions over social networking patents.
Yahoo has been struggling to keep pace with competitors like Facebook and Google in recent years. The company fired CEO Carol Bartz last September and has been overhauling its business strategies.