Facebook's "modern messaging system" may make it convenient to seamlessly move between instant messaging and a Facebook.com e-mail account, but not if you are sharing a link to a file sharing site.

Facebook began blocking BitTorrent link-sharing on Facebook walls and news feeds last spring, and also started blocking private messages between users that included a link to torrents on the Pirate Bay.

Facebook says that content censorship policy isn't changing, even as its new Facebook Messages service gives users e-mail accounts and encourages them to communicate even more through Facebook.

"We have systems in place to prevent abuse on Facebook and prevent spam which we’ll continue to deploy with the new Messages," a Facebook spokeswoman said in a written statement. "We don’t share specifics on those systems."

When a user tries to share a Pirate Bay link (including those to copyright-free content) in a message, Facebook analyzes the link and returns an error message saying "Message Failed. This message contains blocked content that has been previously flagged as abusive or spammy. Let us know if you think this is an error." The block extends to all Pirate Bay links, including one for a Canterbury Tales file in the public domain.

As a communications service, Facebook isn't liable for the content of any of the messages on its service or even on user's profiles. Facebook's blocking is legally questionable under federal wiretapping laws, according to the Electronic Frontier Foundation. Facebook says it's within its legal rights to block links, and does so to protect intellectual property rights.

But it's an extraordinary step for an online messaging service to take – censoring users on the basis of content. Like other mail systems, Facebook weeds out malicious links and spam.

Facebook has a score for how closely you are "friends" with each of your Facebook friends, and it's likely that the increased messaging on Facebook will be used to tweak those scores. Facebook declined to answer a question about whether it would do so. However, those scores are key to Facebook's News wall, where the company selectively tells you what your friends are up to, so that you see more of what your best friend is up to, than what your long-lost friend from summer camp is doing.

So will Facebook begin using the content of messages to better profile its users?

The company says no.

"Facebook Messages is about person-to-person communication and messages are private," the same spokesperson said. "Facebook is not using any content in messages for its own use."

That's an odd sentiment to square with the company's filtering of messages before they are sent, but that's their line and they are sticking to it.

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