Growth of number of Twitter users according to Twitdir

A while ago I predicted that Twitter would have 1 million users before April 1, 2008. The source for the number of (public) Twitter users is Twitdir. Twitdir reports on its homepage the total number of Twitter users detected.

I observed a spectacular slowdown in the growth of the Twitter users reported by Twitdir. See the graph below.



Is the growth really slowing down ? Or is Twitdir not able to catch up ?