"If it's free you are the product" is up there with "you know Google once said 'don't be evil'" in the list of vapid clichés in 'critical' tech writing. Open software is free. Are users of any given open source system the product? Not really, eh? How about readers of free books and articles? Yeah, just stop repeating that. Being free isn't the problem. The business model of the entity offering the good is the problem.
That's a good point!
One distinguishing factor is whether what is offered is a service rather than an information "good". With goods you are usually not the product.
There are free services though: archive org, Wikipedia.
One additional test is whether there is a mandatory signup process to use the service. Do they exist at scale?
There are free services that require signup where you are still not being marketed to, which exist at scale: mastodon! We need more of those.