A secret experiment on 700,000 Facebook users to see if moods could be manipulated has led to an apology of sorts from the social network.

The study was carried out by researchers from Cornell University and the University of California, with oversight from Facebook's data scientist Adam Kramer.

In the study users' News Feeds were tampered with, according to a report on the Proceedings of the National Academy of Sciences (PNAS) journal. PNAS said positive or negative posts were removed to see if it had any impact on the mood of the community.

The publishing of the report has led to shock and concern about privacy, but Facebook said the experiment happened a long time ago, and had no major impact on users.

"This research was conducted for a single week in 2012 and none of the data used was associated with a specific person's Facebook account. We do research to improve our services and to make the content people see on Facebook as relevant and engaging as possible," said the firm.

"A big part of this is understanding how people respond to different types of content, whether it's positive or negative in tone, news from friends, or information from pages they follow. We carefully consider what research we do and have a strong internal review process. There is no unnecessary collection of people's data in connection with these research initiatives and all data is stored securely."