Ferrell and McKay, the guys that run the company, are based out of Hollywood - meaning they have a wealth of wannabes at their disposal. The exposure, especially now that FunnyOrDie is on HBO, is probably worth working pro bono if you're not an A-List actor (sorry Olivia Wilde, you're not).
Most of those guys are folks that have traveled in Ferrell and McKay's social circle over the years, so it's kind of a friendly thing.
While it's cool that these folks slum it for the Internet, the downside is that it makes an already skeptical industry even more hardened in its belief that the Internet is for kids and small little YouTube videos like the ones on FunnyOrDie.