Facebook privacy hoaxes just won't die. It seems like every few months, some extended family member or high school "friend" will post a big block of legalese-style text, urging others to share it in their own feeds with a promise that it will somehow protect their privacy.

Two different ones are circulating now. One claims that Facebook will now start charging a subscription fee in order to keep posts private -- unless the user copy and pastes the message advertising the fee into their Facebook status.

But, of course, the company is planning no such thing. And Facebook has fact checked similar posts that have gone viral in the past.

So why do the hoaxes keep coming back?

Perhaps newer users haven't been shamed by their Facebook friends for posting them yet. But James Grimmelmann, a professor at the University of Maryland School of Law who's researched Facebook privacy in the past, said that the hoaxes fit into a tradition of viral digital media, harden back to the infamous e-mail from the late 1990s that promised Bill Gates would give people money for forwarding it.

Both the Gates email and the Facebook privacy hoaxes rely on users not really understanding how the services work. "The idea that Bill Gates was actually monitoring every email sent was implausible -- but it was just plausible enough to people using e-mail that it still got sent around," he said.

The Facebook privacy or copyright posts may be obvious hoaxes to those who closely watch the tech giant (like, say, tech reporters), but they seem at least possible to some users because of how often the social network has tweaked its privacy policy and user interface through the years, according to Grimmelmann.

Faced with even the slightest possibility of dire consequences, users may decide its worth ten seconds to post the messages.

Users may also have trouble judging the credibility of things they read on the social network, Grimmelmann said. "Unless you look closely at the credit line, Facebook itself strips away some of the contextual cues you might traditionally use to differentiate actual news from viral content."

Since people are now conditioned to getting at least some of their news through Facebook, why wouldn't they learn about a change to the social network on it first? And even if someone isn't sure where one of these hoaxes started or hasn't done their own research on them, they may have felt safe assuming it was true if it was promoted by someone they trust online.

But if you're thinking about writing up an irritated status update to correct your hoax-sharing friends, be gentle: They're probably just trying to help.

"There's an expressive feeling of participation in something that appears to be important and significant -- and in this case, might give voice to users who have frustrations with the platform," said Grimmelmann.