When Picasa Web Albums first introduced the community search, the numbers it returned for popular queries were crazy – giving numbers in the hundreds of millions, as noted on

http://blogoscoped.com/archive/2007-03-12-n67.html

Now things seem to have settled down, and numbers returned by searches seem to be much more accurate.

The top two words in my experiements with just over a million results each are "the" and "and" (nothing all that surprising there!) I've extrapolated from those results to guess that Picasa Web Albums may be hosting around 30 million photos by now. See

http://www.zmarties.com/picasa/blog/2007/03/picasa-web-albums-learns-to-count.html

for my reasoning.