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Geo-networked Facebook users are being warned to check their privacy settings after photos of U2 frontman Bono and a couple of bikini-clad girls were made available to the entire New York network.

IT security firm Sophos says it called on the wildly popular social networking site last year to review its default security settings after it discovered that joining geographic networks over-rode other privacy settings.

When users joined a location-based network, their entire profile was opened up to others on the network, no matter how serious previous settings were.

On a network the size of New York's Facebook population, this is far from ideal.

According to reports, in this latest incident, one of the girls involved, Andrea Feick, posted her private photos to the social networking site.

Since 19-year-old Feick was a member of the NYC network, over a million people were able to see the photos, which featured Bono and friend Simon Carmody and another teen, Hannah Emerson in St Tropez.

"This could all be completely innocent on Bono's part, but it raises the serious security issue about how everyone must ensure their identity is properly protected online," said Graham Cluley, senior technology consultant at Sophos.

"Facebook's privacy features are more sophisticated than some competing social networking sites, but the fact that Facebook changes these without asking when you join a geographic network is a huge oversight. While in this instance the result is potentially embarrassing for Bono, other members of the site could easily find themselves the victim of identity theft."

Cluley advises Facebook users to exercise caution when joining these networks and to ensure they reset their security settings, and keep potential cybercriminals away from their personal information.