By Miguel Helft and Jessi Hempel

FORTUNE — On a Friday morning not long ago, Mark Zuckerberg gathered his troops for a much-anticipated all-hands meeting at Facebook’s brand-new headquarters. It was billed as a not-to-be-missed event. Employees who were traveling were encouraged to return to the mother ship, and those in New York, Dublin, Hyderabad, and other satellite offices were told to watch via live stream. In Menlo Park, Calif., some 2,000 employees marched into a large white tent that was set up for big gatherings on a lawn across from the parking lot. The mood was effervescent, or as one employee described it, “part religious revival.” The pre-meeting buzz had people betting that Zuckerberg was finally ready to discuss the momentous event that would transform the eight-year-old company from hot startup into card-carrying member of the business establishment: Facebook’s IPO.

Zuckerberg had other plans. He mentioned the IPO only in passing. This was a gathering to discuss priorities for 2012, according to employees. In a sense, the purpose of the meeting was to remind everyone to stay on course even as Facebook prepared to undergo its biggest change yet. No matter what happens on the outside, Zuckerberg told employees, keep your heads down. “Stay focused,” he urged. “Keep shipping.” Twelve days after the January staff meeting, Facebook announced its plans to go public.

This story is excerpted from the March 19, 2012 issue of Fortune. To read the article in its entirety, please buy it on Amazon here or at Barnes & Noble here. This issue of Fortune will be on available on newsstands and on tablets starting Saturday, March 3. Print subscribers to the U.S. edition of Fortune can access the iPad edition FREE. To become a print subscriber, click here.