PALO ALTO, California: Advertisers call it 'behavioural targeting', media watchdogs deem it an 'invasion of privacy'. But call it what you will, online social network finds itself mired in controversy with a new feature that shows users what their 'friends' have been up to on partner websites.

The advertising program, aka Beacon, has caused disquiet among privacy campaigners such as Civic Action, which urges Facebook members to sign a petition protesting the move.

Beacon can be used by online retailers to track the spending habits of Facebook users on their sites. When a user makes a purchase, a message is sent to the news feeds of their friends, telling them what they bought.

Facebook says its members are made fully aware of what is being tracked and can opt out if they wish.

But Move MoveOn spokesman Adam Green believes this is not enough.

"The opt-out is very well hidden. It basically pops up for a second and then goes away, and it's on the bottom of your screen when you're purchasing on a totally unrelated website, so you aren't even looking for it," he complains.

In addition, members have to repeat the process on each partner site: "Even if you see the opt-out and jump through the hoops of opting out once, that doesn't solve the problem."

Facebook insists the opt-out is a robust safeguard, but chief privacy officer Chris Kelly insists the network welcomes feedback and could change its settings and policies for advertising and privacy.

He claims the reaction from users so far has been "fairly muted".

Data sourced from Wall Street Journal Online; additional content by WARC staff