Facebook Inc. Chief Executive Officer Mark Zuckerberg said Wednesday that he made a “huge mistake” in not focusing more on potential abuse of users’ personal information, as the social-media giant he founded revealed that data breaches were far more extensive than previously known.

Mr. Zuckerberg’s most direct mea culpa to date came as Facebook disclosed that data from as many as 87 million of its users may have been improperly shared with an analytics firm tied to the 2016 campaign of President Donald Trump, up from the...