While the vast majority of people may use Facebook as a platform to share pics of their kids and grandkids, recipes, funny memes, TV show recommendations, and other completely harmless posts, it's also a platform for those who want to spread bad information around.
Among the more stubborn and harmful misinformation out there has been considerable false claims regarding vaccines, most notoriously their utterly debunked and completely unsubstantiated link to autism. Vaccines do not cause autism, as several major studies have confirmed.