Facebook has countered Yahoo's patent infringement claims with some patent infringement claims of its own.
It's countersuing the stricken old Internet queen in federal court in Silicon Valley, charging it with infringing 10 of its patents.
According to the New York Times it's enough to cover 80% of Yahoo's revenues last year. Including display advertising, content personalization and photo sharing, that adds up to $4 billion.
Facebook, whose users increased Yahoo's traffic by 300% in Q4, is ticked that Yahoo would turn on it and bring suit just before it IPOs at a gargantuan $100 billion valuation.
It wants the court to rule Yahoo's broad, if not sometimes vague, fundamental patents invalid. (Unfortunately, perhaps, for Facebook they're not all vague.) And of course it's demanding damages.
Yahoo allegedly treads on Facebook patents relating to the social network's newsfeed and digital media tagging.
It's unclear if Facebook is using patents it got recently from IBM.
There are copies of the suit in the Times, VentureBeat and All Things Digital.
Yahoo could reportedly lay off 25% of its people Wednesday morning.