Academics at the Cambridge Psychometrics Centre published an influential paper in 2013 about predicting personality from Facebook likes, the result of research which had been under way for some years.

The BBC has seen a 2011 email from Prof Eckles, then a researcher at Facebook, to the Cambridge academics expressing interest in their work.

He describes undertaking similar research.

"We have been using analysis of linguistic data to infer personality traits," he wrote.

"We have a manuscript that is going through some revisions now."

Two years later in 2013, another Facebook employee wrote to the Cambridge academics, apparently unaware of the previous contact.

Srikant Ayyar wrote that he had read their recent paper with interest and continued: "Our group does similar work with the goal of improving our product for people who use it and advertisers. …..we are growing to keep pace with Facebook growth and growth in the data we collect."