It's also a popular idea in the wider world of business, which predates FB by several thousand years. If you are interested in an abstract theory of wealth, that's great. But the rest of the world transacts in money, which is brought in as revenue and paid out as expenses. At the risk of oversimplifying, when you subtract the latter from the former and the number is greater than 0, then you can make a claim of profitability. Users donating data and attention might be good for your business model and help increase profits indirectly. But they are fundamentally different from customers who pay money (a means of transferring wealth) to use your product or service (a form of wealth).
Counter-example: Barnes and Noble does not consider users who visit their stores to read books and magazines without paying for them as customers. They provide B&N with incredibly valuable data (e.g. which books and magazines people want to read) and eyeballs, but they aren't customers. And neither are Facebook users.