Looking for huge MySQL applications

Does anyone know of any existing operational web sites where a massive (ie. millions of records) MySQL database combines with PHP to provide a directory service? I'm thinking along the lines of the White or Yellow Pages or similar. I ask because I'm researching a project which provides a (large) web-based specialist directory service, and I'd like to be in contact with people who've already done it with MySQL and PHP. I use MySQL and PHP all the time for small applications, but I'm not sure how they compare (in terms of speed and reliability) in the commercial world against the expensive alternatives. Thanks for your help.

At www.mysql.com they mention running mysql with up to 50 million records.
http://www.mysql.com/php/manual.php3?section=Features