A group of influential Democratic senators called on Facebook Inc. to quickly end the postings of firearms for sale on its popular Marketplace and demanded information about how it polices gun sales on the service.

In a letter to Chief Executive Mark Zuckerberg, 15 senators—including three 2020 presidential candidates—asked for information about the company’s knowledge of, and efforts at policing, the sale of guns on Marketplace. Last month, The Wall Street Journal reported sellers of firearms were clandestinely posting guns...