In February 2018—as rumors swirled about Facebook’s interest in supporting cryptocurrencies in its apps—one of the company’s top executives, David Marcus, told a gathering of venture capitalists in Los Angeles not to hold their breath about such a plan.  

Mr. Marcus, the former president of PayPal who had recently joined the board of the crypto exchange Coinbase, downplayed the consumer appeal of existing cryptocurrencies like bitcoin and ethereum, criticizing the costs and slowness of using them to facilitate payments. But he didn’t shut the door to Facebook eventually getting into the business, saying “maybe we’ll do something” once the crypto industry matured. His needle-threading allowed Mr. Marcus to postpone questions about privacy and other issues such an initiative would likely raise at a moment when public scrutiny of Facebook was intensifying. 

Mr. Marcus will finally have to answer those questions on Tuesday, when lawmakers in Washington, D.C., begin grilling him about Libra, the digital currency that Facebook finally unveiled last month. The Senate Banking and House Financial Services Committee hearings this week are expected to be tense, judging by the criticism the company’s plans have prompted from everyone from President Donald Trump to the U.S. Federal Reserve Chairman Jerome Powell.