I guess it's sort of like a fast-food restaurant owner telling the customers that the food isn't healthy. What's the point? Everyone probably already knows that

I guess it's sort of like a fast-food restaurant owner telling the customers that the food isn't healthy. What's the point? Everyone probably already knows that, and they'll eat it anyway (for various reasons). Facebook (and other social media sites) is like the digital version of a fast-food restaurant. And luckily for them they have 2 billion users eating it, so there is a lot of peer pressure on others who'd rather not eat to go there anyway because their friends are there.