At Facebook headquarters, the Q&As work like this:
Each Friday at 4 pm PT, Zuckerberg speaks for about an hour from the cafeteria in Facebook's massive new building in Menlo Park. Zuckerberg's top lieutenants — folks like COO Sheryl Sandberg, product boss Chris Cox and CTO Mike Schroepfer — sit in the front row of chairs set up for employees in case Zuckerberg wants to call on them to answer a question.
The meetings are usually limited to Facebook employees, though others have made appearances, too. Board members Peter Thiel, Susan Desmond-Hellmann and Don Graham have all attended Q&As, as did Jay Z back in the summer of 2013, though no one seems to remember why he was there. If you work out of a remote Facebook office, the Q&As are streamed live and then uploaded to Facebook's internal portal for a short time so people can watch them at their convenience.
Zuckerberg starts with opening remarks, then typically acknowledges any long-tenured employees celebrating a work anniversary that week — what Facebookers call a "Faceversary." If you've been there long enough, usually around a decade, you might get to go up and share a favorite story about your time with Facebook.
Then Zuckerberg highlights a "fix of the week," usually honoring some behind-the-scenes engineering fix or accomplishment that others may not be aware of. It's a small shout-out that's meant to help Facebook keep its "hacker DNA," even as the company swells in size.