Impressive numbers from Facebook on their architecture: 1,800 MySQL servers (900 pairs of master/slave) holding a heavily partitioned data set managed by just 2 DBAs.
Failures are handled by promoting a slave to master and then putting in a new slave quickly. It all must be heavily scripted and automated to be managed by just two people. Very nice.
Please see also the garbled but tolerable video from the panel, "Scaling MySQL -- Up or Out?", from the 2008 MySQL Conference where these numbers were disclosed.
I have to say, YouTube looked pretty bad sitting up there on the panel refusing to provide any numbers at the same time Facebook was being so forthcoming. But, Cuong Do Cuong's talk last year, "YouTube Scalability", from the Google Scalability Conference has some nice details on YouTube's architecture and lessons learned, though no specific numbers.
Please see also comments on the numbers from James Hamilton and Om Malik.