Well then, let me perform some of my own analysis. Seems to me the traffic to Vonage's web site was down in part because of fewer-signups. That phenomenon would have driven site visits down because fewer Web users would have gone to Vonage.com to research the service as well as consult the various tech and customer support resources assembled there.
And as to the existing Vonage customers carried forward from December, 2006's flurry of visits?
Not so much that they have left Vonage as they have:
Become more used to Vonage, and have less need to go on to the Vonage site for questions:
Have become more aware of independent third-party resources such as the Vonage Forums, and have become more comfortable asking questions (and venting) there.