Facebook is going after marketing firm uSocial, the company that claims to sell 'fans' on the popular social networking site to its customers for marketing purposes.

According to uSocial, which also sells Twitter followers and more recently started selling YouTube views, Facebook lawyers have sent a cease and desist warning to the firm.

The letter claims that uSocial is breaking several laws, including illegally accessing the site. However, although uSocial has now backed down on the selling of 'friends', it is digging its heels in on its right to sell 'fans' for marketing purposes.

"We have been dealing with Facebook regarding this matter and we have removed our 'friend' buying service, though we're not budging on selling 'fans'," said uSocial chief executive Leon Hill. "We are well within our legal right to do so and, despite the cease and desist, we are not taking the service down."

This is not the first time that uSocial has been in trouble over the services it provides. The firm received a similar cease and desist note in 2008 from social bookmarking site Digg.

"We are actually not too worried by this, and we will be ramping up our Facebook services in the coming weeks," said Hill. "We'll shortly be launching a similar service with Facebook groups, though we will have to wait and see what the response is from that."