A parade of regulators, politicians and law enforcement officials demanded to know more about Facebook’s privacy practices on Monday, as the fallout from the company’s relationship with a political data firm continued to spread. Early in the day, the Federal Trade Commission confirmed reports that it was investigating how Facebook handles information about its users. Soon after, Senator Charles E. Grassley, Republican of Iowa and chairman of the Senate Judiciary Committee, invited Mark Zuckerberg, Facebook’s chief executive, to testify about privacy standards next month. He also extended invitations to Google’s chief executive, Sundar Pichai, and Twitter’s chief executive, Jack Dorsey. And a group of 37 attorneys general sent Mr. Zuckerberg a letter asking for details about Facebook’s privacy safeguards. But one group of people interested in Facebook — investors — had a muted reaction to the growing regulatory threat. Shares of Facebook fell sharply after the F.T.C. confirmed its investigation but ended the day up 0.4 percent as overall trading on Wall Street recovered from a big drop last week. Read more at The New York Times.