Lots of things can be harmful or useful based on how you use it. You can learn many useful things from the internet while there are people looking for child porn and recruiting terrorists. You can use a car to get places quicker or run people over. You can use nuclear power to generate clean electricity or make bombs.
A lot of people including me use Facebook services to stay in touch with my friends and family most of whom live thousands of miles away from me, which I find incredibly useful. People use FB groups to form a community around lesser known causes (like a rare disease or a dying language) and get in touch with people they might never have found otherwise. At the same time, Facebook can be used in harmful ways like you mentioned.
Sure, Facebook can perhaps regulate people using it's services to protect "fragile" people, but don't people deserve the freedom to do whatever they want with their time? Should we also have laws around how long you're allowed to use the internet every day because it's addicting and can be harmful?