If you're offered a chance to install a "dislike" button on your Facebook page, ignore it. It's part of a phishing scam.

The idea behind the "dislike" button is to allow users to flag comments they don't like. Facebook has a "like" button but not a "dislike" button, and says it has no plans to add one.

"It's the latest survey scam spreading virally across Facebook, using the tried-and-tested formula used in the past by other viral scams including 'Justin Bieber trying to flirt,' 'Student attacked his teacher and nearly killed him,' 'the biggest and scariest snake,' and the 'world's worst McDonald's customer," Sophos security expert Graham Cluely wrote in his blog.

The hook comes in a post that is supposed to look like it came from one of your friends. It says "I just got the Dislike button, so now I can Dislike all of your dumb posts.lol!!!" Other versions say something like "get the official Dislike button now!"

Becoming vulnerable

The scam targets victims to download an application that allegedly will install the Dislike button on their page. Instead, the link downloads a rogue application that gives the scammer access to the victim's Facebook account.

The program also installs a message on the victim's page, urging friends to also install the button and providing the link. Cluley says, in that way, the scam is growing virally.

The main purpose of the scam is to allow scammers to post spam messages on millions of Facebook pages.

A spokesman for Facebook said the site tries to make sure links and rogue applications are taken down as quickly as possible.

"We always encourage people to not click on links that appear suspicious -- even if posted from a friend," a spokesman told the BBC.