It would be disingenuous for Facebook CEO Mark Zuckerberg to ignore the 42.6% decline in his company’s stock price since it went public in May. But on the day when Facebook says it recorded its 1 billionth user, Zuckerberg remains upbeat — and denies that he’s over his head — in an interview with Matt Lauer to be broadcast tonight on Rock Center With Brian Williams. Here’s a clip that ran this morning on Today: