With Metcalfe's law, bitcoin is indeed quite promising, as the users have been increasing and are expected to increase further.
This means the value of bitcoin is increasing. Could this argument be valid?
We do not know whether the number of "bitcoin users" is increasing.
The sources that I know (such as blockchan.info) do not give that information. They give some quantities (such as wallet software downloads, transactions per day, total BTC volume per day) from which some people claim to be able to derive the number of users. However, those quantities include an unknown amount of operations that do not imply real additional use. Some of them (like total BTC volume) have been relatively constant for the last 6 months. Moreover, there is no information at all about people who stopped "using" bitcoin, e.g. after buying a bit just for curiosity.