If there's one thing likely to stir Facebook users into action, it's news that their privacy might be being further eroded by the social network.As I wrote at the end of last month on my blog at Sophos.com, Facebook is planning changes to its privacy policy that would give a green light to third-party sites accessing users' personal information.

These changes mean if you're logged into Facebook and then visit a third party Website, that site will be able to access the following:

Hmm.

In a nutshell, you might visit a Website and discover it already knows who you are, your date of birth, where you live, and who your friends are -- all without you ever having given the site explicit permission to access that data.

Unsurprisingly, Facebook users who are aware of the planned change to their privacy are up in arms, as the following poll conducted on the Sophos Website reveals:

Facebook privacy poll

Even though Facebook says only a small number of preapproved sites will be offered this feature, and that users would be able to "opt-out," an overwhelming 95% of the 680 people polled believe the changes to be a "bad thing."

And it's not just people polled by Sophos who are unimpressed; German Consumer Protection Minister Ilse Aigner is reported to have sent an open letter to Facebook CEO Mark Zuckerberg complaining about the slackening of privacy and threatened to quit the site in protest.

Once again, it feels like online privacy is being eroded by stealth. Too many Websites are chipping away at their members' privacy and security, potentially exposing their personal data to third parties that were never in the equation when they first signed up for the service.

In my opinion, an "opt-out" option simply isn't good enough -- users should have to consciously opt-in to having their data shared with others

Think carefully about the personal information you are posting information on social networks -- you can't be sure who they might be sharing it with tomorrow.

More details about the Facebook privacy poll can be found on my blog on the Sophos website.

Graham Cluley is senior technology consultant at Sophos, and has been working in the computer security field since the early 1990s. When he's not updating his award-winning other blog on the Sophos website, you can find him on Twitter at @gcluley. Special to Dark Reading.