Keyani says this phase of the hackathon is one of the most fun--there are always a few "ah-ha" moments. "We'll see a presentation and say, 'Why haven't we shipped this yet?'" he says.

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After the prototype phase, Zuckerberg and other executives choose a couple of projects to view more in-depth, Keyani says, and they'll give the groups feedback and suggestions for making it better. From every hackathon, there are one or two projects that go on to become a new Facebook feature.

The follow-up phase is one of the most important--and it sets the tone for future hackathons, Keyani says. Employees need to know that the business takes their ideas and commitment seriously. "Otherwise it just disrupts people's schedule and they view it as, 'We spent time working on stuff, then dropped it,'" Keyani says.