Facebook's Big Announcement
1:07 PM/ET: Zuckerberg says "our mission is to make the world more open" and "if we give people tools" they'll be able to connect more.
People think about Facebook for staying connected, Zuck says. But originally people used Facebook for "Making new connections"
1:05 PM/ET: Sheryl Sandberg just showed up and is making her way to the front. Here comes Zuckerberg!
1:03 PM/ET: The music turned off. It's standing room only.
There is still ZERO indication of what the news is, but there are a ton of reporters here, particularly those who cover mobile news. No doubt this announcement involves mobile.
But then again, anything Facebook does involves mobile.
Will the news live up to the hype? Why did the stock just suddenly drop? Where's Zuckerberg?
(Read More: Is Facebook's Ride Over?)
12:45 PM/ET: We're eagerly awaiting Facebook's mystery announcement. The social network sent out an invitation for the press to "come and see what we're building" and about 120 bloggers and reporters have turned out.
Now we're all in the "King of Ping and Pong" room in building 15, and there's still no indication of what the announcement is about. The signs on the walls say "Move Fast and Break Things," "Fortune Favours the Bold," "Focus," and "What Would You Do if You Weren't Afraid."
(Read More: Facebook Charging $100 to Message Mark Zuckerberg)
Here's what I know: It's not a physical building. It's not a phone hand set. It doesn't involve a big partner-- there's no sign of any other company here. It's some sort of product for users, not something for advertisers.
And it surely is all about MOBILE and will have implications for Facebook's bottom line. My bet: some sort of mobile video chat app, a change for Instagram, or some sort of app store/game platform.
(Read More: One Thing We Know Facebook Is Working On.)
Unlike many previous announcements, Facebook is not live streaming this event, nor is the company letting broadcast crews in. Why the high security? The company reminded me that back before the IPO it held a number of no-camera event like this. But things have certainly changed since Facebook went public, and the attempt to keep things quiet inevitable leads to more speculation.
(Read More: Where Facebook Is Looking to Grow: COO Sheryl Sandberg.)
And with the stock's run-up in recent months the stakes are higher than ever for the event to live up to expectations.
(Read More: Video Ads May Be Coming to Your Facebook Feed.)
—By CNBC's Julia Boorstin; Follow her on Twitter: @JBoorstin and with reporting by CNBC producer Harriet Taylor