UPDATE 2:53 p.m. PST with comment from Yahoo

This is getting ugly.

Just three weeks after after Yahoo filed a patent infringement lawsuit against Facebook, the social giant struck back on Tuesday with a counterclaim alleging Yahoo is infringing on 10 of Facebook's own software patents.

“From the outset, we said we would defend ourselves vigorously against Yahoo’s lawsuit, and today we filed our answer as well as counterclaims against Yahoo for infringing ten of Facebook’s patents,” said Ted Ullyot, general counsel of Facebook, in a statement. "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 prioritize litigation over innovation."

Facebook's counterclaim (.pdf) is similar to Yahoo's original complaint. It alleges that Yahoo infringed on methods of providing basic web functionality such as search, headline feeds, photo tagging, and, of course, advertising.

The software patent skirmish comes at a sensitive time for both companies, though in opposite ways. Yahoo filed its lawsuit just six weeks after Facebook announced its planned IPO. Facebook filed its counterclaim Tuesday, the day before Yahoo is expected to enact massive job cuts. AllThingsD was first to report the lawsuit.

Yahoo responded in a statement e-mailed to Wired:

We have only just received Facebook’s answer and counterclaims, but on their face we believe they are without merit and nothing more than a cynical attempt to distract from the weakness of its defense. As we have made clear from the outset, the unauthorized use of our patented technology is unacceptable and must be resolved appropriately. Other leading companies license these technologies, and Facebook must do the same or change the way it operates. We have proposed that Facebook join us in discussions to resolve the matter, but our overtures have been rejected. As a result, we are prepared to continue to seek redress through the courts.

When Yahoo -- newly helmed by former PayPal CEO Scott Thompson -- sued Facebook last month, it was widely seen as the ultimate trolling act -- an expression of sour grapes from an ailing company bleeding marketshare, revenue and talent at an alarming pace, all to Facebook's benefit.

It likely stems from a partnership Yahoo formed with Facebook nearly two years ago, when Facebook deeply integrated its services into Yahoo's website.

The idea was, rather than fight Facebook head on by denying its patrons access, Yahoo would make its own site more robust by making Facebook more accessible via Yahoo's services. E-mail notification updates, messenger client access, even status messages were accessible via Yahoo accounts. “We are creating a world where we are letting people share Yahoo content and give people access to whatever is relevant to them wherever we are," Yahoo’s senior director of social platforms Cody Simms told Wired last year.

Instead, the plan backfired on Yahoo. People found it easier to cut out the middleman, and flocked to Facebook, bolstering the network to a user base that draws closer to 1 billion each day, while leaving the ailing Yahoo to figure out what kind of company it wanted to be. Was it a social network? A "content company," as former CEO Carol Bartz used to so often trumpet? A search engine? Even Yahoo executives couldn't figure it out, and began leaving in droves.

Facebook's countersuit, then, looks less like another round of patent trolling, and more like a defensive move against Yahoo, a once-great company struggling for a second life.