The reasons I've seen are:
> it creates a risk of bad self-image for young girls
It's a parent's job to educate your children. There are much worse things than Facebook out there.
> it collects data
Literally no harm in knowing that someone is interested in JavaScript, cats and fetish porn, and targeting ads to that user.
> it's addictive
So is sex, marijuana, and collecting stamps.
> it helps organize protests
Good.