Children do not have fully formed brains. Society has decided the best way to mitigate the negative consequences stemming from decisions made by children with brains which have not fully developed is to allow parents the authority to make decisions for them.

If an adult wants to spend all day in front of a computer screen, they can choose to do so. If an adult wants to spend all day on Facebook, they can choose to do so.

If you believe Facebook would turn you into a bad person, you are well within your right to not use Facebook.

I don't use Facebook, but that's because it's an unenjoyable experience. Not because I think it's some evil hypnosis device.