https://www.theverge.com/2018/6/7/17439316/facebook-posts-bug-public-shares-14-million

Facebook said today that it was informing 14 million users that they may have accidentally shared posts publicly because of a bug.

The company disclosed today that the bug, which was live last month, switched the “suggested” sharing setting on some users’ posts to the “public” setting. Facebook generally sets its suggestions for sharing based on users’ settings and previous activity. But in this case, a software error caused the default setting to be set to public, regardless of whether users had previously chosen to share with a smaller group of people.