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I'm guessing they're talking about unit tests and integration tests, the integration tests probably simulating user input using a framework such as Selenium(RC).

It's been a while since I've had to deal with it (bootcamp), but I'm pretty sure we still use Watir [1]

[1] http://watir.com/

Others have already mentioned Selenium, but another integration test framework is CasperJS.

http://casperjs.org/

can't speak on facebook specifically, but one popular option is Selenium http://seleniumhq.org/