Facebook has made good on its vow to fight Yahoo's patent claim by filing a counter suit also alleging patent infringement.
The company said that Yahoo was infringing on 10 of its patents related to the operation of its web services and social networking patents.
Additionally, the company issued a categorical denial of a series of patent infringement claims filed by Yahoo.
Facebook has long said that it would be contesting Yahoo's claims.
"From the outset, we said we would defend ourselves vigorously against Yahoo's lawsuit, and today we filed our answer as well as counter-claims against Yahoo for infringing 10 of Facebook's patents," a Facebook spokesperson said.
"While we are asserting patent claims of our own, we do so in response to Yahoo's short-sighted decision to attack one of its partners and prioritise litigation over innovation."
In a copy of the suit provided to V3, Facebook said that Yahoo services, ranging from the Yahoo homepage to the company's Flickr platform as well as its Games and Sports sites, make use of technologies that infringe its patent rights.
Facebook alleges that the infringing services account for some $4bn in revenues.
In particular, Facebook alleged that Yahoo's services violate its patents on news feeds, tagging media files and posting headlines.
Eight of Facebook's patents concerned in the lawsuit were acquired as part of a recent deal with IBM. The other two were developed internally, including one attributed to company founder, Mark Zuckerberg.
The filing is the latest in what has become an epidemic of patent infringement suits within the consumer and enterprise IT industries.
Recently, entrepreneur Mark Cuban suggested that a major decision in the case between Yahoo and Facebook could provide an impetus for much-needed reforms in the patent system.