Like most business or project metrics a single number is meaningless.  Metrics become valuable in the aggregate where you can track numbers over time or where you can take 3 or 4 different metrics to spot trends or do other analysis.  As Nik said, we actively track a lot of different metrics from a wide variety of sources.  They pretty much all tell the same story.  The number of users and active DotNetNuke installations continues to rise month after month.  Even something as innocuous as a blog post by a leading book publisher provides yet one more touch point that is consisitent with every other metric we have seen.  So while someone may quibble over whether there are really 400K or 350K users on DNN.com or wether we really have over 3.3 million downloads ( the number is actually larger since our GotDotNet numbers don't show up in the SourceForge stats), what is indisputable is that DotNetNuke continues to see growth in adoption and usage that is on par or better than the vast majority of open source projects.

The below stats represent a count of users who have logged in at some point since the given date.

ActiveUsers as posted on 3/1/2007
Since Jan 1, 2005 - 331,220
Since Jan 1, 2006 - 228,787
Since Jan 1, 2007 - 45,288

ActiveUsers as of 6/5/2007
Since Jan 1, 2005 -- 371,470
Since Jan 1, 2006 -- 270,943
Since Jan 1, 2007 -- 99,140

Some additional stats that will help put things in perspective

UserLogins by Year
2005 -- 100,527
2006 -- 171,803
2007 -- 99,145

Also, remember that this does not necessarily represent all DotNetNuke users since many people setup DotNetNuke sites without ever visiting DotNetNuke.com, or without ever logging into DotNetNuke.com.

While the numbers are interesting I think that it is too easy to make assumptions about what the data means.   I do believe that it shows the community is growing.  At the current pace we should hit about 220,000 users having logged into DotNetNuke.com during 2007.