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We do not know whether the number of "bitcoin users" is increasing.
It is quite a surprise that there's no concrete information confirming increasing users, though news is flying daily for more adoption!
Giving it a careful consideration, I agree Jorge's comment. Anyone have different opinion.
If users are not convinced to increase, Metcalfe's law is not valid for bitcoin. How do we get to be convinced for increasing users?
It's a pseudonymous currency, of course we don't know the number of users or adoption rates. And we likely never will unless people volunteer to answer surveys or something similar.
We do know that outside corporations seem to believe adoption is growing, or possibly that they wish for it to grow. The evidence is increasing adoption by businesses. Why did Visa state that they could incorporate bitcoin into their system? Why did Global Payments allow bitcoin payments to their clients? Either a) increased adoption is happening, or b) they want increased adoption to occur.