We don't actually have a reliable way to measure the number of bitcoin users.  The Metcalfe Value plots I prepare assume that it's correlated with the number of TXs per day and the number of unique addresses used per day.  The plot below shows that both of these two proxies for the number of users has indeed been increasing exponentially (along with bitcoin's market cap).  Today it appears that a "concave down" function would fit better than a straight line, but late last November people were arguing for a concave up (super exponential) model.   So we must wait and see.  The ups-and-downs away from exponential growth look like noise to me and are not yet significant enough to invalidate the model.