Issuing a warning to the Guardian Media Group ahead of its publication of an exposé of mass Facebook data harvesting was not the wisest move, one of the social networking giant’s senior executives has said.

Addressing the FT Future of News conference, Campbell Brown, head of news partnerships at Facebook, said: “If it were me I would have probably not threatened to sue the Guardian,” adding it was “not our wisest move”.

GMG received a legal letter from Facebook the day before the Observer reported that a company harvested as many as 50m Facebook profiles of US voters for Cambridge Analytica and used them to build a powerful software program to predict and influence choices at the ballot box.

Brown’s comments come after Facebook’s founder, chief executive and chairman, Mark Zuckerberg, acknowledged that the policies that allowed the misuse of data were “a breach of trust between Facebook and the people who share their data with us and expect us to protect it”.

Brown, a former journalist with CNN and NBC, reportedly told the conference on Thursday: “It’s been a rough week. Frankly, it’s been a rough year.

“Inside the company and outside the company, people are very upset. I’m upset. This breach of trust that’s happened as Mark described it last night doesn’t feel good for anyone.”

The crisis stems from Facebook policies that allowed third-party app developers to extract personal data about users and their friends from 2007 to 2014. Facebook greatly reduced the amount of data that was available to third parties in 2014, but not before a Cambridge University researcher named Aleksandr Kogan had used an app to extract the information of more than 50 million people for Cambridge Analytica for commercial and political use. There was no connection with Cambridge University.