That's a popular quip, but I think it's actually a form of thought-terminating cliche. If you want, they're clients in the sense that a government torturer's victim is their client.

I think that whoever is paying for something is the true client.

I also think that calling something thought-terminating can, itself, be thought terminating. In this case, a way of dismissing a commonly made point under the assumption that I have not thought through the issue on a much deeper level, but I have done so. We may simply disagree on our conclusions.

Users of facebook are paying for something, they're just not paying for it with cash. They're paying for it with their personal data.

That's not revenue. From Facebook's perspective, that's an input, a resource they have to obtain. Users are a supplier of raw material that Facebook distills & refines and sells on to the customers that pay them actual money.

Like any business, they have to maintain a good relationship with their suppliers. In this case they have to provide a platform that users return to over & over again. But they optimize their business towards revenue generation, not suppliers.