Sheryl Sandberg with Elliot Schrage, Facebook’s outgoing head of communications, who has taken the blame for hiring Definers. “I’m sorry I let you all down,” he said

Sheryl Sandberg has changed her account of how much she knew about a PR firm that Facebook recruited to conduct smear campaigns against opponents.

The chief operating officer of the social media giant said last week that she did not know it had hired Definers, a Republican-affiliated consultancy, or “about the work they were doing”.

She has now admitted that she was copied into a “small number of emails” that referenced the business and that some of its work was presented to her.

The New York Times reported last week that Definers encouraged journalists to investigate ties between the billionaire George Soros and other critics of Facebook. Mr Soros’s office has since called the tactic a smear campaign that has echoes of antisemitism.

Definers also