Debian Linux, Firefox, gcc, visual studio... The list goes on. I think the "you are the product" quip has become an overplayed meme of late. Mutually beneficial / co dependence and such themes are more applicable in more cases than not when it comes to free products and services, in my own view anyways.
The quip is certainly overplayed, but I think the notion still stands. It is important to be aware of what intentions service providers and product sellers have. It's not so much a dig on libre / free software.
This took literally two seconds to think of off the top of my head. The quotation is _still_ not accurate. It's a useful perspective on how to think about free services, but it's not a substitute for thought.
Other services like that are public radio and TV.
That doesn't apply to most services like facebook for which each active user creates extra bandwidth and processing load.
It's been overplayed for years. The problem with pithy phrases, even when true in their original narrow sense, is that they allow those who aren't willing (or capable) of thinking for themselves to substitute something that superficially looks like wit without having to actually think about what they're talking about.