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Your photos - stuck in the cloud?

• Rory Cellan-Jones
• 21 May 09, 11:12 GMT

The into what happens - or doesn't happen - when you delete those embarrasing snaps from Facebook and MySpace tells us a couple of interesting things.

The first is obviously that you're bonkers to put anything online that you don't want a future employer, partner or aged relative to see - because, if the experiment is to be believed, that embarrasing shot of you in fancy dress at a stag night will remain online even after you've deleted it.

The second is that big social networks like Facebook and MySpace are struggling to cope with a difficult dilemma - balancing the costs of running their networks with their users' demands for privacy.

Facebook insists that photos are removed immediately from its own computers when users delete them, but admits that they take longer to disappear from its content delivery network.

When I sounded puzzled about this the PR person pointed me at a which explains that a content delivery network is "a system of computers networked together across the internet that cooperate transparently to deliver content to end users."

What Facebook, MySpace and other big websites do is use these content delivery networks to store and distribute all those millions of photos and videos which their users want to share. So you may think you are giving your snaps to one computer in California owned by Facebook - but the Cambridge research appears to show that the photos are actually stored on servers maintained by an external company which runs the content delivery network.

Eventually, the deleted photos will disappear from the photo server's cache. Facebook does explain this process in its terms and conditions, as the report from Cambridge points out, telling users that the deletion process is "similar to emptying the recycle bin on a computer."

In other words you may have put your pictures in Facebook's bin, but you will still have to wait for the content delivery network to delete them. True, it is quite difficult to find the "deleted" photos - the researchers worked out their location by carefully noting their URLs before deleting them - but your pictures are still out there in the cloud.

In a telling phrase, the , where the Cambridge researchers outline their experiment, says the strategy adopted by Facebook and other social networks is understandable because "photos are deleted from these types of sites too infrequently to justify the overhead and complexity of removing them from the content delivery network."

In other words, it's all about costs - something the social networks are struggling to control as their user base expands far more rapidly than their revenue. Facebook in particular has been very successful in bringing in new users around the world. The trouble is that every time a user in, say, India uploads a picture, that costs Facebook just as much money in infrastructure costs as a picture uploaded by an American user. Yet the Indian Facebooker is going to bring in even less advertising revenue than his American counterpart.

Social networking is booming around the world - but the business model still looks shaky. What the Cambridge experiment has shown is that networks like Facebook and MySpace have decided that they just can't afford to give users as much privacy as they might like. And that means that entrusting your photos to the cloud can mean relinquishing control of the way you appear online.