When it comes to protocols and formats, using the word "proprietary" adds more confusion than clarity.
Consider:
H.264 has a spec that's written down by a standards-setting organization and not a trade secret (though behind a paywall) and has multiple independent interoperable implementations. Yet, it's "proprietary" in the sense that it's patent-encumbered. I.e. the patent holder are the proprietors.
VP8, OTOH, is Royalty Free with a Free Software canonical implementation and has other implementations, too, though their independence is debatable. Yet, VP8 is called "proprietary" by some, because the design of VP8 was under a single vendor's (Google's) control and not blessed by a standards-setting organization.
I think using the word "proprietary" as the opposite of "free as in Free Software" is fine when talking about a particular implementation, but it's better to avoid the word when talking about protocols and formats.
For protocols and formats, it's more productive to talk about:
* Royalty-free vs. encumbered
* Multiple independent interoperable implementations vs. single implementation.
* A Free Software implementation exists vs. doesn't.
* Fully specified vs. defined by a reference implementation.
* (If fully specified) Non-secret spec vs. secret spec.
* (If non-secret spec) Spec available at a freely GETtable URL vs. spec behind paywall or similar.
(A number of Googly things that are royalty-free and have a Free Software implementation go to worse end of these axes on the points of having a single implementation and being defined by the quirks of that implementation.)