The hackathon: it's a regular event at Facebook, during which employees build prototypes for ideas they have. The company says that several of its key products came out of the exercise. But last night's hackathon was special given the unique circumstances. CEO Mark Zuckerberg is pictured here giving a pep talk to several thousand Facebookers gathered on the eve of the company's monster IPO. Click ahead for more scenes from Hackathon 31.