You already know Yahoo is suing Facebook, claiming its "entire social network model" was made possible by Yahoo's patented Web technologies. Now, it turns out Yahoo is indicating it may assert another 16 patents against Facebook that Yahoo claims are being used in open source technology Facebook adopted for its data centers and servers.

In a Securities and Exchange Commission filing yesterday, Facebook said "We received a letter dated April 23, 2012 from Yahoo indicating that they believe 16 patents they claim to hold 'may be relevant' to open source technology they allege is being used in our data centers and servers. Yahoo has not threatened or initiated litigation with respect to matters described in this letter but it may do so in the future."

The letter from Yahoo has been reprinted on ZDNet and TechCrunch. It reveals the 16 specific patents in question, with Yahoo claiming Facebook's Open Compute Project is the source of much of the alleged infringement. The April 23 letter requests a meeting with Facebook by April 26. Facebook's SEC filing yesterday doesn't say whether such a meeting has taken place.

Among the 16 new patents, one describes a "method for garbage collection in an object cache," several discuss methods of serving up Web content from Web servers, and another discusses a server farm cooling system. Yahoo's suit against Facebook was filed March 12, citing 10 patents related to social networking, privacy, messaging, and customization. Facebook fired back with a counterclaim alleging that Yahoo infringed 10 Facebook patents, only to see Yahoo file a further counterclaim last week alleging two more.