I don't know that I agree with completely cutting a kid off facebook at 17.
The goal is at 18 (maybe 21) that they will be functioning adults.
Let's assume you think functioning adults should know how to use Facebook very judiciously & sparingly, then your goal should be that by 17 your kid should be using Facebook judiciously & sparingly, by now with minimal guidance from you.
Yeah, theory. Mine are raised.