Here's an interesting link about "Who has the most web servers?" It only briefly touches on guesses for the number of servers Google and Microsoft have, but they provide interesting numbers for companies like 1&1, Rackspace, and a few other big names, based on this "Netcraft hosting provider server count".
As an interesting note, Rackspace and 1&1 both seem to claim to have more than 50,000 servers, but that can't be validated by the Netcraft numbers.
As a final interesting note, all of the numbers reported by Netcraft show data centers growing, and in some cases, growing very rapidly. It makes me wonder what the world's electrical power consumption looks like compared to the time before the internet?