If two or more parties are communicating via a standardized protocol, then using that standard constitutes an implied agreement between the parties communicating.

Whereas with Facebook, while one or more parties might be bound by some agreement with the company, that doesn't create any clear agreement between users.

This is a pretty weak argument. Using a standard established by an RFC implies a user has read that standard about as much as using Facebook implies a user has read the TOS. The TOS establishes a transitive condition that the user agrees to about as much as they agree to one-to-one commumication under the RFC.

If you have a problem with usage implying consent to a TOS then you can make that argument, but this one is flimsy.