There are many other intangible assets that produce money but are not easily sell-able: Trademarks, human capital, trade-secrets, other intellectual property, etc.
All of these would be in Ford's side without any doubt. Maybe profit-per-capita is the single benchmark they'd clearly lose at, and the fact that their workforce is largely unionized, which is a serious risk in times of hardship (they'd find themselves unable to cut expenses significantly, as it has happened many times in the past).
Ford vs Google might be a close one in terms of IP - hard to compare across industries though - but vs Facebook it's quite one-sided for Ford.
You are correct these things can be viewed as assets. However, if the author is deriving his data from Facebook's own declarations, then even they aren't claiming the value of their assets including User-loyalty, are anywhere near their market cap.
Auditors would stick to the the accounting standards and conventions to give a monetary value to users and other intangible stuff. Whereas the market (in theory) can value the whole company's ability to use these intangibles to generate money in the future. That's why there's often a significant difference between the "assets - liabilities" on the balance sheet, and the market cap. Now, whether such a big discrepancy is justified... that's another thing.
Under GAAP, there are a lot of intangibles that are not reported on the balance sheet. Basically all of facebook's internally developed intangibles are not on the BS, but all of Instagram's intangibles will be there. Kind of weird, but this is how purchase accounting works.
Users, pageviews, etc, are assets. If the market will be able to determine their "correct" value... that's another question.