Social network Facebook has admitted it hired a PR firm to plant negative stories about Google in the media.

The strategy came to light after blogger Chris Soghain published an email conversation he had with Burson Marsteller after the PR agency approached him with a story on behalf of an unnamed client about Google's Social Circle service.

"The American people must be made aware of the now immediate intrusions into their deeply personal lives Google is cataloguing and broadcasting every minute of every day - without their permission," Burson told Soghain when discussing Social Circle.

The web was awash with rumours over the identifiy of the unnamed client, with everyone from Microsoft and Apple being named. However, US website The Daily Beast soon discovered it was in fact Facebook behind the smear campaign.

The social network told the website it had used the PR agency to highlight activities by the search engine that "raised privacy concerns".

Neither Facebook or Google would comment on the story.