Facebook’s mantra for a long time was “move fast and break things," but one thing a sharing-based platform shouldn't break is users' trust. The social network apologized Wednesday for a controversial “news feed” study, where some 700,000 users were unwitting test subjects.

“This was part of ongoing research companies [advertisers] do to test different products, and that was what it was; it was poorly communicated,” said Sheryl Sandberg, Facebook’s chief operating officer, according to the Wall Street Journal. “And for that communication, we apologize. We never meant to upset you.”

The study was done in 2012 on nearly 700,000 users who selected U.S. English as their primary language. Researchers changed the algorithm that produces Facebook’s news feed from users’ posts, so that some users had posts containing positive language shuffled out of their feeds and others had negative posts removed. Besides not informing users that their feeds were being adjusted, Facebook was criticized in part for purposely making users sad by removing positive posts.

While the social network’s terms of service may protect it from any lawsuits in the U.S., Facebook is under investigation in the U.K. to see if it violated data-protection laws there. The Information Commissioner’s Office (ICO) is a watchdog that monitors the use of personal data in the U.K. and can levy fines and force policy changes for companies found to be breaking British law.

“We take privacy and security at Facebook really seriously,” Sandberg said. Social media companies have a vested interest in how much users share, with bad privacy or security policies causing them to share less.

Facebook told International Business Times on Tuesday that it had no record of which users were a part of the study, which was published in the March issue of the “Proceedings of National Academy of Sciences.” Sandberg is in India this week visiting the company’s offices and meeting with smaller advertising partners as Facebook prepares to launch ad campaigns tailored to local markets.