What Facebook already has — more than any other digital company — is a spectacularly rich vault of information about its users, who cannot seem to stay away from the site. Americans, on average, now spend 20 percent of their online time on Facebook alone, thanks to the ever-growing menu of activities the company has introduced, from playing games to sampling music to posting pictures of baby showers and drunken escapades. Some 300 million photos are uploaded to the site daily.

How Facebook exploits its users’ information — and how those users react — is the next reckoning. David Eastman, worldwide digital director for the advertising agency JWT, said Facebook would need to give marketers more data about what kinds of users click on what kinds of advertising, and about their travels on the Internet before and after they click on an ad. Most brands want to have a presence on Facebook, he said, but they do not quite understand who sees their pitches and whether they lead to greater sales.

“They need to make the data work more,” Mr. Eastman said. “They need to provide deeper data. Right now the value of Facebook advertising is largely unknown.”

While the bulk of Facebook’s revenue comes from North America, it is banking on international growth. The company has expanded its global footprint so rapidly that four out of five Facebook users are now outside the United States. It is the dominant social network in large emerging markets like Brazil and India, though it shows no signs of penetrating China — where it would face not only government censorship but stiff competition from homegrown social networks.

Mr. Zuckerberg, who has studied Mandarin, signaled his ambitions to crack the vast Chinese market as far back as 2010. He suggested that Facebook would first try to advance deeper into markets like Russia and Japan before it took on a country as “complex” as China.

With international growth comes international regulatory headaches. Facebook already faces audits in Europe on whether the company is living up to promises made to consumers about how it uses their data — and now, a stringent new data protection law. In India, it has been sued for spreading offensive content. And in the United States, it faces privacy audits by the Federal Trade Commission for the next 20 years. In its offering documents, Facebook repeatedly warns of legislative and regulatory scrutiny over user privacy, “which may adversely affect our reputation and brand.”

Maintaining brand loyalty is excruciatingly difficult in the Internet business. Across Silicon Valley, investors are plotting the next big thing in social networks. Already, the clock may be ticking for Facebook.