A year ago, Automattic (the folks behind WordPress) bought Gravatar which is used around the web by lots of folks of to display pictures, etc. in blog posts and more.
The Gravatar blog has an update which includes some interesting stats on just how heavy the usage of Gravatars is (emphasis added),
Gravatar now lives on about 20 servers: 2 Database servers, 1 File server, 2 Load balancers, 5 Caching servers, 9 Web servers, and 1 Development server. That combination of servers is handling an average of 7,214 of your requests every second of every day. That’s a whopping 623,293,056 requests daily! 96% of all of those requests are served directly from cache. These days we get around 5,000 uploaded images every day. Even with this staggering increase in the number of requests we’ve been able to make Gravatar faster, and more reliable than it’s ever been.