It's free to you, not free to them.

If I'm supposed to let them sell my privacy and attention, I wouldn't really call it free. Money isn't the only thing of value I can give to a website.

Free means you don't have to give them money to use it. If you use a different definition, you will often be confused.

It's the voluntary nature of the payment and whether the cost is tangible to you that determine what is free, not the currency used. Substitute "your FBI file" for your ad network profile and "compute time on your Amazon instance" for JavaScript time in your browser to make this clearer.

Tell me more about this Silicon Valley place where everything is magically free?

I'll have to defer to someone who actually makes that claim.

Decide for yourself, either use it or don't.