I would like to present a different view of the matter.
However, the key question is this: Can the growth of Twitter the service be accurately measured by an outside party?
With a vast number of users now using many different applications to access the service, the number of visitors would appear to be dropping, simply because there is no need to visit Twitter itself while using the service. In order to gain an accurate metric of growth we must turn to Twitter itself, and count the raw number of tweets and the users sending those tweets.
In short, I don't think that it is possible for anyone but Twitter to make statements about their growth (or lack of it). Raw visitor metrics are not enough, even somewhat irrelevant, once the concept of Twitter as a service is acknowledged and understood.