From a Yahoo! article I came to know that the top 3 social networking sites in the US are MySpace.com , Facebook.com and bebo.com. Interestingly, each of these companies uses a completely different middle-tier stack. MySpace.com uses Microsoft-IIS 6.0, Facebook.com uses Apache 1.3.37 and bebo.com uses Resin/3.0.21. MySpace.com uses cfm (ColdFusion?) for it’s dynamic content, Facebook.com uses PHP and bebo.com uses JSP. I am not sure which database is used by MySpace.com and Facebook.com, but bebo.com claims it’s powered by Oracle. Chances are either one or both of the other two may be powered by MySQL. From job descriptions of Facebook.com, they seem to require experience preferably in MySQL and then Oracle. Perhaps, they are using both databases.