Facebook has sent a letter to the U.S. Drug Enforcement Administration demanding that agents stop impersonating users on the social network.

The letter follows a BuzzFeed report that revealed how the DEA seized a woman's phone and later created a Facebook account in her name.

Sondra Arquiett was unaware as the DEA masqueraded as her while speaking to her friends. The DEA even posted photos of her with her son and another photo of her alone in panties and a bra.

She has sued the DEA agent who set up the account. The Justice Department is backing him up, claiming federal agents have the right to do such things.

Now Arquiett has Facebook (FB) on her side.

"The DEA's deceptive actions... threaten the integrity of our community," Facebook chief security officer Joe Sullivan wrote to DEA head Michele Leonhart. "Using Facebook to impersonate others abuses that trust and makes people feel less safe and secure when using our service."

The letter goes on to say that Facebook shut down the DEA's fake Arquiett account. It also demands that the DEA confirm it stopped all other cases of impersonation.

The DEA declined to comment and referred all questions to the Justice Department, which has not returned CNNMoney's calls.

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How did the DEA end up with this woman's phone? In 2010, Arquiett was arrested and faced charges related to cocaine distribution. She pled guilty and received probation.

In legal filings, a federal prosecutor said Arquiett "implicitly consented by granting access to the information stored in her cell phone and by consenting to the use of that information to aid in an ongoing criminal investigations [sic]."

But in its letter, Facebook said it is "deeply troubled" by that legal position.

Privacy researcher Runa Sandvik, who advises the Freedom of the Press Foundation, explained it this way: It's one thing to strike a deal and become an informant. It's another to lose complete control of your online identity.

"Isn't this the definition of identity theft?" Sandvik asked.

This is only the latest case in which the technology firm comes head-to-head with the federal government on civil liberties issues.

Facebook has asked for more transparency about NSA spying on Americans. CEO Mark Zuckerberg has called President Obama to complain about it. And Facebook is fighting in court to have the Manhattan district attorney's office justify its seizure of 381 people's accounts.

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