In a bid to increase member numbers, social-networking giant Facebook is opening up member details so they can be discovered through online search engines such as Google, and Yahoo.

Right now, in order to search through Facebook profiles web users have to be Facebook members, or at least be invited to join the social-networking site by an existing member. But today Facebook told its members that it intends to allow anyone on the internet to search through members. Searches can take place on Facebook itself, and through internet search engines.

Crucially, users have to opt out of the search service, which is enabled by default. This means that Facebook's decision adds to concerns about privacy, recently raised by code leaks from Facebook and the propensity of some Facebook users to post sensitive details on their profiles. In theory, all 39 million members could be visible to search engines when Facebook's changes go live tonight.

To remain invisible to non-members, Facebook profile holders have to opt-out of having a "public search listing". They can also choose to restrict their profile to being searchable only through Facebook's front page.

Members' profiles will not become visible to the public. The search listing available includes only a person's name and photograph. To contact a profile holder, users must first sign-up to Facebook.

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