As explained by The Guardian:
But Franken, angry and sardonic, interrupted: “People are buying ads on your platform with roubles. They’re political ads. You put billions of data points together all the time. That’s what I hear that these platforms do: they’re the most sophisticated things invented by man, ever. Google has all knowledge that man has ever developed. You can’t put together roubles with a political ad and go hmm, those two data points spell out something bad?”
Stretch replied: “Senator, it’s a signal we should have been alert to and in hindsight–”
But Franken cut him off, asking whether Facebook would pledge not to publish a political ad paid for in North Korean won. As Stretch demurred, Franken interjected fiercely: “Please answer yes or no, sir. You’re sophisticated. You’re the chief legal counsel for Facebook. Please answer yes or no.”
Stretch sought to clarify that currencies did not necessarily indicate the source country of an advert and refused to commit to banning political ads purchased in foreign currencies. But Franken snapped back, asking why a bad actor would choose the North Korean won to conceal his activities. He told the counsel: “My goal is for you to think through this stuff a little bit better.”
Stretch couldn’t just say yes.