Hillary Clinton. Getty/Justin Sullivan

Hillary Clinton and Facebook's chief operating officer Sheryl Sandberg are good friends. So much so, Sandberg endorsed Clinton during her 2016 presidential bid and there were rumours she might take up a political post in a Clinton administration.

But all the rumours came to nothing after Donald Trump won, and now Sandberg is answering awkward questions from Congress about how Russia used Facebook ads to manipulate the election.

Speaking to the BBC during a trip to the UK, Hillary Clinton acknowledged her friendship with Sandberg and said she "has not been satisfied" by the company's explanations so far.

Asked by interviewer Jane Garvey whether Sandberg was "embarrassed" by what had happened, Clinton replied: "Well, she is now speaking to Congress. She's trying to explain it to Congress, but I so far have not been satisfied by the explanations from the company."

Earlier in the interview, Clinton described Trump's as a "new kind of campaign, a reality TV campaign."

She said voters "have to be really careful and even skeptical about news [they] get on social media. Facebook is now the largest platform for news in the world."

Chief operating officer of Facebook Sheryl Sandberg. Stephen Lam/Reuters She went on: "They have to answer the questions and come clean, they have to share the ads, they have to reveal all the different sites that were operated by Russians posing as Americans, foreign money coming in. They owe it to our country. I don't let friendship stand in the way of what I believe is important to the national interest."

On Monday, Clinton described Russian interference as a "cyber 9/11."

Sandberg last week gave an interview to Axios in which she carefully answered questions about how Facebook had been used during the election campaign. The company had earlier revealed that Russia spent $100,000 on Facebook ads which were intended to sow discord and division.

That included buying ads related to the racial justice movement Black Lives Matter, and impersonating Muslim groups.

Sandberg described the interference as "troubling", and said Facebook was keen to publish the Russian ads publicly. US Congress will publish the ads once Facebook has scrubbed any personal information from them.