Not sure this should be a surprise, but Facebook sniffs email content exchanged on the built-in messaging platform, then censors that content. I haven't tested this.
As outlined in the post, it's not unusual for content to be deconstructed algorithmically, it's done all the time. And email shouldn't be considered private by any stretch. But the fact that Facebook is then censoring that data wholesale amps up the discussion about Facebook falling out of favor with some.
Wired.com ran a test using a torrent file linked on the Pirate Bay to confirm:
Wired.com confirmed Facebook is blocking private messages by sending a link to a Pirate Bay torrent feed of a book in the public domain. Such content is freely available to everyone, as all copyrights have expired. Nevertheless, the message bounced twice, returning the following failure notice: “This Message Contains Blocked Content. Some content in this message has been reported as abusive by Facebook users.” (Facebook’s link-censoring system is may be just tilting at windmills, however, because removing a single vowel from the domain name lets the URL go through.)
In the case of Wired.com’s test, there were only two Facebook users who should have been aware of the content — Wired.com editor John C. Abell and his message’s intended recipient, who was sitting five feet from him — and neither had the slightest objection to it whatsoever.
The whole post is worth reading.