Updated for the full year 2013
In May (2013) I produced a set of graphs and numbers on our contributor growth based on analysing the regular changeset dumps. Given that this was half a year ago I believe it is time for an update.
Our total contributor number has continued to grow at something around 7'000 per month giving a total of 391'367 at the end of December:
The number of active contributors per month continues to show a slight upward trend: The jump in April 2009 coincides with anonymous edits no longer being allowed.
Which reflects itself in the number of active contributors per year too (the year end number for 2013 is 129'000).
The above graph also shows that the number of contributors that contributed in a previous year is showing growth too. Note the lack of "old" editors going from 2008 to 2009 is likely due to anonymous edits going away in 2009. This is likely also the reason for other low counts in the early years.
And while it is a bit too speculative to state reasons, we do see, compared to active contributor growth, over proportional growth in the number of changesets. Going over 500'000 per month for the first time in September of this year: