New statistics on editing frequency, produced by User:Dragons flight, show that the size of the active editing community of the English Wikipedia peaked in early 2007 and has declined somewhat since then. Like Wikipedia's article count, the number of active editors grew exponentially during the early years of the project. The article creation rate (which is tracked at Wikipedia:Size of Wikipedia) peaked around August 2006 at about 2400 net new articles per day and has fallen since then, to around under 1400 in recent months.
Corresponding data for the number of active editors was available from Erik Zachte's Wikipedia Statistics site, but due to the large size of the complete database dump, the numbers for English Wikipedia have not been updated since October 2006, just when Wikipedia appeared to be entering a new phase of growth. The new data shows that the community continued to grow for about six months after the peak in article growth rate, reaching a maximum of 18,126 registered user accounts (excluding bots) with at least 20 article namespace edits in the month of March 2007. By September 2008 (the last month covered by the new statistics) only 13,971 accounts made 20 or more article edits. Anonymous edits and total non-bot edits across all namespaces also peaked in March 2007.
The proportion of active users who contribute heavily has remained relatively stable since the peak of activity in 2007, although active users were more likely to have high edit counts during Wikipedia's period of very rapid growth in 2005 and 2006. In 2005, users making at least one article edit in a given month were about twice as likely to make over 100 edits that month than in 2007 and 2008. However, the proportion of active users making at least 2500 article edits per month has been rising since early 2007.
User:MBisanz has charted the number of new accounts registered per month, which tells a very similar story: March 2007 recorded the largest number of new accounts, and the rate of new account creation has fallen significantly since then. Declines in activity have also been noted, and fretted about, at Wikipedia:Requests for adminship. In 2008, only 201 editors were granted adminship, compared to 408 in 2007; in December 2008, there were only nine successful requests for adminship, and on December 31, there were no active requests. However, the number of active administrators (which appears to have peaked just above 1000 early in 2008) has seen only slight decline; as of January 2, 2009 there are 944 administrators who have made at least 30 edits in the past three months.