Devil's Advocate

Free software does not usually spy on its users because the users, who are in control, do not usually want it. The only exception I am aware of is Ubuntu's dash. Because access to the source code is granted to every user, a spyware cannot easily be hidden. If it is, the one who did it made a very risky move: his reputation is a stake. The malware can be detected by any user taking a close look at the source code.

Although it is perfectly OK to do so, free software is usually not sold "per-copy". The main business model is to sell support (insurance that user problems are solved within days or even hours, custom developments, configuration and integration in an information system, user formation, etc.). You can take a look at this study (in particular pages 24-25) to know more about free software business models. Contrary to building a large database infringing the users' privacies, there is nothing wrong with those business models.

I would answer your question with an answer based on this three facts:
1. facebook is a company who wants to make money
2. there is no other way for them to get money with their current business model than to collect data
3. Maximizing the amount of money grace to unethical practice can not be prevented by anybody - no one can control what they are doing!

Point 2 and 3 are different when we're talking about free software developers.
Point 1 can be.
That's the difference why "you are the product" doesn't apply in this case.

I was thinking more along the lines of - how do you identify free thinkers and those likely to be objectionable?
Give them a flag to gather under.

Hmm, this may be a bit too paranoid.

"if you're not paying for the product, you are the product".

If the product is free, you will be the owner of your copy.

It's better to say

"You're not their customer but their product."

Free Software is a community, or an adjective, or a movement, not a product.

The product is whatever is sold with it: support for example. "Give away the razor, sell the razor blades."

I would rather be the product than having someone telling me what product I'm going to be.