A new report says Facebook altered the News Feeds of hundreds of thousand of users as part of an experiment into human behavior.

The company's on-staff researchers manipulated the amount of positive or negative posts for nearly 700-thousand users to study how they would react through the social network.

Data manipulation is written into Facebook's Terms of Use.

People who bother to read the terms when they sign up for Facebook agree that their information may be used "for internal operations" which include data analysis and testing research.

The study was published in the "Proceedings of the National Academy of Sciences."

(Photo: Justin Sullivan / 2011 Getty Images)