Yahoo Inc. (YHOO) returned legal fire against Facebook Inc. Friday in the companies' ongoing patent dispute, calling Facebook's recent countersuit slapdash and asserting two new patents against its younger peer.

Yahoo sued Facebook last month, alleging the social site infringed ten of its patents related to online communication, ads and privacy features.

The lawsuit caused a stir in Silicon Valley, where many tech firms adhere to a philosophy of only using patents for defensive purposes, in order to avoid stifling entrepreneurship and innovation.

Yahoo's suit also added some drama to Facebook's impending initial public offering of stock, which is expected next month.

Facebook countersued Yahoo earlier this month, asserting that its own ten patents had been infringed. Those patents included one naming Facebook Chief Executive Mark Zuckerberg as an inventor, while others had been acquired.

Facebook has been ramping up its patent acquisitions in the wake of Yahoo's lawsuit, paying Microsoft Corp. (MSFT) $550 million for a patent portfolio while purchasing hundreds of other patents from International Business Machines Corp. (IBM).

In its answer to Facebook's counterclaim Friday, Yahoo says Facebook bought eight of the patents it used to countersue from "non-practicing entities," or firms set up to develop, buy or assert patents rather than use them to make products.

Those entities are sometimes referred to as "patent trolls."

All eight of the Facebook patents were purchased in the past five months, Yahoo says, and several were purchased after Yahoo had filed suit.

Some of Facebook's patents are "tainted," Yahoo alleges, and its infringement claims are based on "conjecture."

In addition, Yahoo asserted two new patents against Facebook in its legal filing Friday. Those patents covers ways of analyzing interaction between online contacts and of managing online search advertising.

A Facebook spokesman said in a statement, "We remain perplexed by Yahoo's erratic actions. We disagree with these latest claims and we will continue to defend ourselves vigorously."

A Yahoo spokesman said in the company's own statement that its filing Friday "underscores the breadth of Facebook's violation of Yahoo's intellectual property."

-By John Letzing, Dow Jones Newswires; 415-765-8230; john.letzing@dowjones.com

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