I use Hacker News without (as far as I know) being their product.
I pay for Cable TV and yet I'm still their product.
It's not as clean-cut as that; you need to read the small letters and always use caution, paid service or not.
Your HN usernames are required on the YCombinator app forms. There is a good chance that they will go through your comment history to get a feel of your personality. I would think that this figures in their "buying" process (they are buying a part of your company after all). If you look at it like that, you are the product.
Of course if you support a site, you'll be happy to help promote it. But what happens when you change your opinion on a matter that becomes important/illegal in the public eye, without the option to remove your now-offending content? Or if you plain just want to stop supporting a site? If people are guarded when making comments because of this risk, is the free service improved or worsened?
Email, picture or opinion, make no bones about it - our content is always product in some way. Content lock-in should rarely be tolerated. I'm surprised it is here, TBH. I happen to greatly appreciate this community, but I in no way agreed to give ownership of my thoughts in return for the right to interact. I'd rather pay with a content export/removal option than hand over the sum total of what little wisdom I have.