How do these websites conduct performance testing? Do they use any open source load generator such as Jmeter / Grinder? Or do they have their own (have their own food?)
Not exactly Facebook / Twitter, but Atlassian has unveiled some of its performance testing methods that they use for JIRA and Confluence, you can read about them here
Twitter seems to have built its own iago challenge. It looks pretty decent, especially if you have avi points.