New York State Attorney General Andrew Cuomo said on Monday his office has subpoenaed Facebook, accusing the social networking site of not keeping young users safe from sexual predators and not responding to user complaints.

In a letter accompanying a subpoena for documents, Cuomo said a preliminary review revealed defects in Facebook's safety controls and in its response to complaints. He said the shortcomings contrasted with assurances made by the company.

In recent weeks, N.Y. state investigators have went undercover to test Facebook's safety controls and procedures, posing as underage users. The investigators were solicited by adult sexual predators and could access pornographic images and videos, Cuomo said.

A spokeswoman for Facebook, based in Palo Alto, California, said the company was aware of the subpoena and was preparing a statement.

Cuomo's subpoena came amid a joint 50-state investigation into Facebook, News Corp's MySpace Web site and other online social networks. Such sites have come under scrutiny over concerns they may fall short in protecting young users.

Connecticut Attorney General Richard Blumenthal, North Carolina's Roy Cooper and Ohio's Marc Dann said they met with Facebook last week.

We are negotiating with Facebook -- including a productive, face-to-face meeting with Facebook representatives last week in my Hartford office, Blumenthal said. Facebook has a long way to go before we are satisfied.

Blumenthal told Reuters in July that his office had learned of at least three convicted sex offenders who were on Facebook's Web site, adding that that number may be the tip of the iceberg.

Facebook has seen a rapid expansion of users since it expanded beyond its origins as a college student network to include older users and high school students.

Cuomo's letter, which was addressed to Mark Zuckerberg, the 23-year-old founder and chief executive of privately held Facebook, accused the site of failing to match its improvement in safety measures with its expansion ambitions.

It appears that Facebook has not significantly altered its representations about safety and inappropriate content on the site, said Cuomo.

It does not have the right to represent that its site is safe and that it promptly responds to complaints when such statements are not accurate.

Separately on Monday, the Wall Street Journal reported that Microsoft Corp was in talks to buy a minority stake in Facebook that could value the company at $10 billion or more in total.

Citing sources familiar with the matter, the Journal said Microsoft sought to buy up to 5 percent of Facebook for $300 million to $500 million.

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