Ironically, as is usually the case, the news is being spread on Twitter with the hashtag #Facebookdown.
Facebook Will Be Back Soon
Facebook is down for required maintenance right now, but you should be able to get back on within a few minutes. In the meantime, read more about why you’re seeing this message. Thanks for your patience as we improve the site. #facebookdown— Kevin Weatherly (@ikevinax) March 13, 2019
Instagram also seems to be affected.