Facebook seemed like the perfect way to publicise Mark McAndrew's online fundraising project - until he began to suspect that many of the fans of his Facebook page were fakes.

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Channel 4 News has exposed the black market in Facebook endorsements, and showed how users' accounts had been hacked into and used to "like" products in which they had no interest.

Now it's emerged that those fake likes are costing genuine businesses thousands of pounds.

Mark McAndrew is one example. He needed thousands of people to join Charity Engine, a new online fundraising project - so Facebook seemed like the perfect way to publicise it.

"It seemed to be the best-value marketing thing we had ever seen. we absolutely adored Facebook. It was the answer to our prayers," he said.

Fake 'likes'

Charity Engine aims to raise millions of pounds for charity by linking up processing capacity in PCs and selling all that unused computing power. To recruit the participants they need, they paid to advertise on Facebook.

Facebook runs adverts on the right-hand side of users' pages. Every time a user clicks on that ad to "like" the page, the advertiser - in this case Charity Engine - gets charged a fee by Facebook.

Charity Engine spent £10,000, the whole of their marketing budget, and got over half a million likes. They thought it was a bargain until they tried contacting their fans to ask them to download the app. They then began to suspect many of their fans were fakes.

Many had few friends, had posted little personal information, but liked tens of thousands of pages.

Read more: how hackers are hijacking your Facebook 'likes'

'Really suspicious'

Mark said: "We started looking at our fans and found fans who joined in the last week and already liked 5,000 pages.

"They'd been liking football clubs like Arsenal, Chelsea, Liverpool and Manchester United all at the same time.

"There were loads of really suspicious ones."

Every time one of these fake profiles clicked on Charity Engine's advert, Mark got charged by Facebook. But he says there is no value to him in having his ad seen by fake users

Culture of real identity

Mark has asked Facebook to refund all the money he was charged when fake profiles liked his ad - they haven't responded to his to complaints

Facebook told us: "If advertisers suspect that likes are coming from bogus accounts, we would encourage them to report these to Facebook. Facebook is based on a culture of real identity and we work hard to remove bogus accounts using a mix of our reporting tools and our review systems."

Dan Hagen, head of planning at Carat, which advises advertisers on internet marketing, said: "Facebook has constantly faced challenges around its viability as an advertising platform.

"They've grown very quickly because it was an open place where they tapped into human behaviour and people found it great to communicate with each other.

"They're now trying to add advertising in as seamless way as possible, and that's a challenge. Issue with fake likes is just one of many issues Facebook face."