Facebook were able to carry out the experiment because all users have to tick
a box agreeing to their terms and conditions. These include "internal
operations, including troubleshooting, data analysis, testing, research and
service improvement."

In the study, the authors point out that they stayed within the remit of the
agreement by using a machine to pick out positive and negative posts,
meaning no user data containing personal information was actually viewed by
human researchers.

The study, carried out over a week in January 2012, was carried out for
scientific purposes. But evidence that social media can have such a strong
impact on people's mental state will certainly be of interest to advertisers.

The lead scientist, Adam Kramer, said in
an interview when he joined Facebook in March 2012 that he took the
job because "Facebook data constitutes the largest field study in the
history of the world."