SAN MATEO, Calif. (USA TODAY) -- Silicon Valley's latest parlor game this week is what Facebook is up to on Tuesday, when it announces news at its headquarters in nearby Menlo Park at 1 p.m. ET.

As the markets opened Tuesday, Facebook shares were down slightly to $30.85 after investors scooped up shares the day before to push the stock above $30.

The early money is on search engine-related news, given its high financial stakes. Google's share of the overall U.S. mobile ad market - 57%, compared with Facebook's 9% - is largely because of its search business.

Facebook might also make a more aggressive push into gaming or leverage its $1 billion acquisition of photo-sharing site Instagram.

Other possibilities include new ad formats, mobile ad advancements and an external ad network, says Kevin Lee, CEO of Didit.

Facebook raked in about $4.2 billion from advertising last year - 84% of its estimated $5 billion in total revenue, according to eMarketer. Leading the charge were mobile display sales. Facebook took home about 18% of the U.S. market last year - or $339 million - besting Google's 17%.

Google took home 15% of the nearly $15 billion U.S. display ad market, with Facebook close behind.

Payments accounted for Facebook's remaining $800 million in 2012 revenue, eMarketer estimates.

Long odds are on a long-rumored smartphone announcement coming Tuesday despite mild protestations from CEO Mark Zuckerberg at a September public interview with Michael Arrington.

The mobile space is a particularly bedeviling proposition for Facebook, which has been late to the market even as Americans continue to eschew PCs for smartphones and tablets.

EMarketer estimates 70 million people in the USA regularly access Facebook from a phone - about half of Facebook's active domestic audience.

"The marketplace has room for a Facebook phone," telecommunications analyst Jeff Kagan said in a note last week. "Today, only Apple and Google are heavy hitters."