When Yahoo sued Facebook earlier this year, claiming the social networking giant infringes 10 of its patents covering things like personalized advertising and news feeds, the Silicon Valley technorati blew a gasket, accusing Yahoo of violating the "hacker ethic." Companies should innovate, they said, not litigate.

But Yahoo has now taken its Facebook spat a step further, sending a letter claiming that Mark Zuckerberg and company may be infringing on 16 other Yahoo patents in using several open source technologies inside the data centers that underpin their massive social network. This doesn't just violate the hacker ethic. It ventures into the realm of extreme irony.

Yahoo's online empire, you see, is built atop data center technology patented by Google.

On Thursday, in a filing related to its initial public offering, Facebook revealed that Yahoo recently sent it a letter saying that 16 Yahoo patents "may be relevant" to the hardware and software running inside Facebook's data centers. Yahoo declined to comment on the filing, but Wired has seen a copy of the letter, and it specifically points to four open source technologies used by Facebook, including its Open Compute Project data center designs, its HipHop platform for speeding software code, its Tornado web server, and memcached, a data-caching platform that was developed outside of Facebook.

"We also understand that at least one former Yahoo! employee who had responsibility in connection with Yahoo!’s data centers now has responsibility over the operations of Facebook’s data centers," the letter reads. Dated April 23, the letter also asks to discuss the matter with Facebook before April 26. It's unclear whether this deadline was met.

In its filing, Facebook makes it clear that Yahoo has not actually threatened a second lawsuit. But clearly, Yahoo is using the patents as an attack of some sort, and this is quite a turnaround for a company that was the driving force behind Hadoop, one of the most important open source projects of the past decade – and one that was based on patented Google technology.

In 2005, Yahoo joined forces with an independent software developer named Doug Cutting, who was building an open source version of MapReduce, one of the key technologies underpinning Google's massive web operation. Google had published research papers detailing MapReduce, and these served as the basis for Cutting's creation, which he called Hadoop, after his son's yellow stuffed elephant.

By January 2008, Hadoop was driving Yahoo's search engine, and today, it drives all sorts of services across the company's online empire. But the technology soon outgrew Yahoo. Following Cutting's lead, Yahoo continued to develop Hadoop out in the open, sharing the code with the world at large, and the platform was soon driving big-name operations such as Facebook, Twitter and eBay. Over the past half decade, as Yahoo struggled as a business, it could always point to Hadoop as evidence that it still deserved to be mentioned among the giants of the web.

Google eventually won a patent for MapReduce. But three months later, it granted a free license to the Hadoop project, assuring Yahoo and so many others that they weren't in danger of legal action over the patent.

But this hasn't stopped Yahoo from waving its own patents at all sorts of open source software and hardware used by Facebook. Facebook believes this is not just an attack on its business, but an attack on a wide range of companies and individuals who use the same technologies. "Yahoo’s letter takes aim not just at Facebook but at open source and energy-efficient green technologies developed and employed by countless innovative, forward-thinking companies and engineers," reads a canned statement from the company. "We’re defending vigorously against Yahoo’s current lawsuit, and would likewise do so against any new assertion."

In its letter, Yahoo says it believes three of its patents cover data center designs open sourced by Facebook under the aegis of the Open Compute Project. Some have questioned whether this project should be compared to open source software projects such as Hadoop, but many others see little distinction, including Andy Bechtolsheim, the Sun Microsystems co-founder who has donated his personal time to the project in an effort to encourage collaboration on the designs. "I just want to make sure that the world of openness doesn't end," he told us this week at an Open Compute meeting in San Antonio, Texas. "This is just like people working on Linux. They contribute to the bigger cause. That's what I'm doing."

It would seem that Apple will soon use Facebook's designs in building a new data center right across the street from Facebook's facility in Prineville, Oregon, and Mark Roenigk, the chief operating officer of Rackspace, tells us that his cloud-computing company will use Facebook's designs when building a new facility in either Oregon or northern Nevada.

Yahoo is also waving patents at two open source software platforms developed at Facebook: HipHop, which converts PHP code to C++ in an effort to make software run faster, and Tornado, a new-age web server. But the most curious attack involves memcached, which was developed at blogging site LiveJournal, not Facebook. Unlike HipHop and Tornado, memcached is widely used across the web. Facebook is just one of many big names who use it.

This is another indication that Yahoo is targeting Facebook because it's at the top of the heap. The Facebook IPO could value the company at $96 billion. But it also shows that this is a threat to more than just Facebook. You can call it irony. Or you can call it something else. But it shows that no big-name corporation is above waving its patents at the competition. Right now, nobody thinks that Google would ever sue over its data-center patents. But new executives arrive and companies change. Yahoo did.