"“Congress has subpoena power, of course,” says Al Gidari, the director of privacy at Stanford Law School’s Center for Internet and Society, who previously represented several big tech companies in national security cases.
But the Electronic Communications Privacy Act, when it applies, limits what a provider can disclose to basic subscriber information; anything more would require a warrant, according to Gidari. “That’s why Congress would get less than Mueller, who could get either an order or a warrant,” he said. (Gidari wasn’t convinced the ads count as protected content.)
Ryan Calo, a professor at the University of Washington School of Law, disagreed, suggesting old ads, even if they are no longer public, are not equivalent to stored electronic communications.
“Some legal process is required for FB to turn it over to the govt.,” Jennifer Granick, an attorney for the American Civil Liberties Union, wrote in a tweet."