If you're worried about certain pictures of yourself on Facebook, now is the time to delete them. Security firm Sophos has warned users that the social networking site has enabled facial recognition technology on accounts, without informing affected account holders.

Facebook launched a facial recognition system last December, but only made it available to users within the United States. There's probably good reason for this: privacy laws outside of this country are a lot stricter, and the site probably would have found itself in hot water rather quickly.

The feature was not immediately available to all, but you couldn't opt-out beforehand in order to avoid being tagged. Instead, you had to wait for it to be enabled in order to not participate.

Facebook uses facial recognition technology to prompt your friends to tag you, which means photos of you are more likely to be tagged. After all, people are more likely to tag someone if Facebook pops up with a notification and suggests who that person is.

The new tagging isn't much different from the way things used to be. Friends could always tag photos of you, and you would have to un-tag yourself manually. The difference is that the process is now semi-automated, and some may find this an affront to their privacy.

Sophos' Graham Cluley seems to agree: "The onus should not be on Facebook users having to 'opt-out' of the facial recognition feature, but instead on users having to 'opt-in,'" Cluley argues. "It feels like Facebook is eroding the online privacy of its users by stealth."