Computerworld: Yahoo has already sued Facebook for infringing on 12 of its patents, and now Facebook has revealed that it received a letter from Yahoo warning that it may file lawsuits related to 16 more patents. More specifically, Yahoo alleges that open source software used by Facebook may violate Yahoo's patent rights.
Facebook countersued in the pending case, claiming that Yahoo violates 10 of its patents. However, Yahoo said, "On information and belief, many, if not all, of these patents were acquired by Facebook for purposes of retaliation against Yahoo! in this case."