See this is your problem right here. The very fact that there are 900 Million active accounts destroys your assertion that Facebook isn't "useful"..? Do you believe there is some government entity forcing these people to sign up? We both know the answer is No. They sign up and Use Facebook because they find it useful.
If they didn't they wouldn't bother creating and then maintaining an account.
And that my friend is cold hard fact.
Because something is popular doesn't mean it is useful.
Buy a dictionary.