In order to access the service you need to register. The registration form would not allow you to register without accepting it. They only need to prove that you indeed access the service. That doesn't seem too hard to prove for most services out there, or at least the most popular ones.
- They need to prove that "you" accessed the service, not somebody claiming to be you, somebody with the same full name and rough location as you, etc.
- Websites are often pretty bad at actually requiring ToS acceptance to register. The fact that you've accessed the service doesn't necessarily imply you've accepted anything.
- Even if registration is ironclad, accessing the service won't prove you agreed to any particular version of the ToS. The ever popular amend-at-will clauses never hold up in court, so you really do need to know which version was agreed to.
- ToS are often presented coercively. Maybe you've already signed a lease and moved in, but to actually pay your rent you need to accept an additional one or more third-party ToS because the landlord doesn't accept cash or checks. Maybe you've already paid for your vehicle registration, and after the cash is removed from your account you're presented with additional terms that need to be agreed to in order to receive your tags. Even if you've agreed to some specific contract, that kind of coercion can invalidate the additional terms, even though the party whose ToS you agreed to might not have known about the coercion.
I think you missed the point. They can't prove it was him actually filling out the registration form, only someone from some IP address at a specific time. They can't prove the person who logs in using that registered username/password is the same one that accepted the TOS.
ToSes are mostly useless. They generally contain a line that indicates the terms can change at any time and you accept them by continuing to use the service.
I guess you'll need to convince the judge that there's reasonable doubt that you did not create the Facebook account that you use and that you were unaware of the ToS and not acting in bad faith. Not saying it's impossible, but it's not as difficult to enforce the ToS as it has been suggested. They are certainly not useless.
They are useless in that reading them is pointless since they can be modified and/or enforced arbitrarily. If you violate the ToS, the site might kick you off. If you do something the site doesn't like, they can change the ToS and kick you off anyway.