I notice that lots of the medium-largish sites (from hobbyist BBS's to sites like Slashdot, Wikipedia, etc.) built using this approach are painfully slow even using seriously powerful server hardware. Yet compared to a really large site like Ebay or Hotmail (to say nothing of Google), the traffic levels on those sites is just chickenfeed.
To some extent, I would say it has to do with the servers being used. Slashdot has only 10 servers [1], while Wikipedia has only 39 [2]; Hotmail, on the other hand, has around 3500 [3].
A better comparison to Hotmail is the high-traffic Web site Neopets, which has around 200 servers [4] and uses Linux, Oracle, MySQL (for a few parts of the site), Apache, and PHP.
[1] <http://slashdot.org/faq/tech.shtml#te050>
[2] <http://meta.wikimedia.org/wiki/Wikimedia_servers>
[3] <http://www.securityoffice.net/mssecrets/hotmail.html#_Toc491601826>
[4] Estimated by counting the name of wwwXXX.neopets.com domain names (238). Neopets probably has at least 300 servers if you include non-Web servers.
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