I just updated the BiggestWiki statistics on the Meatball Wiki, http://www.usemod.com/cgi-bin/mb.pl?BiggestWiki
The German Wikipedia is now the world's 3rd biggest wiki. With 25507 articles, it just grew past Ward Cunningham's c2.com, the world's first wiki.
There is always an uncertainty in comparing numbers. There is no implication that bigger is better, and this list doesn't compare quality or purpose. Ward's wiki doesn't support any comma count, but instead the total number of pages is used for this site, an unfair advantage that is allowed out of respect for the elderly. :-)
Wikipedia accounts for 13 of the world's 25 biggest wiki websites, but only two of the six biggest. The other four are: susning.nu (2), c2.com (4), Twiki (5), and Enciclopedia Libre (6). Positions 7-13 are held by a strongly growing field of Wikipedias. The Chinese Wikipedia is new on the list at position 23, having 1504 articles.
For more detail on the size of Wikipedia websites, see http://www.wikipedia.org/wiki/Wikipedia:Multilingual_statistics