Imagine a (very fictional) Yelp user who has 0 money, will never go to a restaurant again in his life, or buy anything at all for that matter. But he has perfect memory and can write excellent reviews of thousands of restaurants he's been to in the past.
Does he have any value to Yelp as eyeball for advertisers? No. If Yelp's audience consisted completely of people like him, no one would ever advertise on Yelp again, no restaurant owner would ever pay Yelp for premium whatever status thing they sell.
But our fictional user can write reviews that are consumed by other users. He is providing real value to Yelp, as a non-eyeball.
In the television case, this guy is completely worthless.
Facebook is a simpler example, because most people are both supplying content and seeing ads. If there were no content, no one would go there to see the ads.
Does he have any value to Yelp as eyeball for advertisers? No. If Yelp's audience consisted completely of people like him, no one would ever advertise on Yelp again, no restaurant owner would ever pay Yelp for premium whatever status thing they sell.
But our fictional user can write reviews that are consumed by other users. He is providing real value to Yelp, as a non-eyeball.
In the television case, this guy is completely worthless.
Facebook is a simpler example, because most people are both supplying content and seeing ads. If there were no content, no one would go there to see the ads.