I am not a lawyer. But I think it depends on which country you live in. In the US, you automatically hold copyright on anything you create, which should include internet forum posts. Contract law can be very complicated, though, and terms of service are a form of contract. Typically you can't sign away your rights to your own creations without some form of compensation, because such a contract would be deemed unfair and not enforceable. A contract is supposed to be an equitable exchange, and it is possible for a court to invalidate a contract, or parts of it, if the terms are too heavily weighted in favor of one party or the other. Being a blanket contract like "terms of service" can also count against the contract, because a contract is supposed to be negotiated between the parties that sign it.
But the truth is, if the various parties to a contract disagree about their obligations, it will be up to a court to decide (if you disagree strongly enough to take it that far). That is the purpose of courts: to settle disputes. Whether you are bound by a contract's terms, or whether it might be thrown out, depends both on the language of the law and the skill of your lawyers (vs. theirs). (And unfortunately, it can also depend on whether you have enough resources to fight over it.)