If she was ready before then, why would you keep her off, and if she wasn't ready, why would you interpret that law as requiring you to let her have an account?
All you'll train into her will be a desire to do anything forbidden. Watch out for drinking unless you introduce it in moderation.
I don't think this is encouraging anyone to desire forbidden things; she desired Facebook because she thought it would be fun, and had to wait because those were the rules.
On the contrary, I don't think it's a good idea to teach 13-year-olds that the rules don't apply to them just because they want something.