Apr 30, 2015 · 4 minutes

Behold Facebook's success on the Internet. One out of every five mobile minutes, Zuckerberg said last week, is spent on Facebook or Instagram. Now, according to Pew Research, we learn that Facebook controls 10 percent of digital ad revenue, including 37 percent of all mobile display ads.

On Wall Street, though, you're only as good as your last quarter. And Facebook's stock is down 5 percent in the past week after missing its revenue forecast for the first time ever. The decline, equal to $12 billion in market value, has less to do with any sense of disappointment than the emergence of a question facing Facebook: After such a smashing success, what does the company do for its next act?

Facebook's stock has more than tripled in the past two years, so investors who stuck with the stock from the IPO or who bought it during its first year of trading have made out well as the company delivered on its promise to drive revenue from its mobile app.

So consistently did Facebook beat Wall Street revenue and profit estimates that analysts fell into the error of over-optimism, setting estimates that Facebook would in time miss. This was inevitable, as Facebook's growth in mobile ad revenue was bound to slow over time. Ad revenue (of which 73 percent was mobile) grew 46 percent last quarter, below the 82 percent in the year-ago quarter.

To keep growth from slowing further, Facebook needs a follow-up act. Tech giants these days can't rely on a single core product anymore - the way, say, Google relied on search for years. They expect other giants to encroach on their core markets and so they lay groundwork into new areas themselves.

More than any company, Facebook exmemplifies this approach, having prepared Instagram and video ads as two new revenue streams. Beyond that, it has Messenger and WhatsApp on tap. And if and when growth in those peter out, it can look to the virtual-reality platform it's building on Oculus.

The catch is, while this plan looks good on paper, it's by its very nature a risky one. Forecasts turn into pure speculative fairy dust once they go beyond a couple of years – moreso in the Internet industry where unintended consequences and unpredictable consumers are the rule. Facebook aims to get around this by recalibrating its plan thousands of times – or reiterating, according to the current buzzword.

Investors have faith in Facebook's ability to achieve its plans, having learned how much it can cost to write the company off too quickly. But it's costing money: as much as $3.2 billion in capital expenditures this year. Facebook's headcount has already grown 48 percent in the past year, with most of the hiring skewing toward longer-term R&D projects. R&D costs rose to 30 percent of revenue last quarter from 18 percent a year earlier.

Facebook's core service is showing signs of slow growth in its older markets. North American monthly active users rose only 4 percent last quarter. But engagement continues to rise more, as daily active users in the region rose 7 percent. And more importantly for investors, revenue per user jumped 42 percent to $8.32.

These days, most of Facebook's revenue growth is coming from higher rates advertisers are willing to pay. Price per ad increased 285 percent from the first quarter of 2014, the company said, although Evan Wilson, an analyst at Pacific Crest, said in a report that price-per-ad rates may have declined from the fourth quarter of 2014. “If this trend holds, we think it could result in a big deceleration in price per ad growth as we progress through 2015,” Wilson wrote.

If that's the case, there are promising signs at Facebook's younger initiatives. Instagram has 200 million daily active users who spend an average of 21 minutes per day on the service. Between WhatsApp and Messenger, Facebook handles more than 45 billion messages per day, or more than 6 messages for every person alive. Messenger alone accounts for 10 percent of mobile VoIP calls and this week began offering video calls in some countries.

WhatsApp, meanwhile, has many more users than Messenger, but by adding video calls Facebook is trying to build Messenger into a more robust platform, hoping that users of the scaled-down WhatsApp may transition over for richer features. It may also give Messenger a video-chat capability for customer-service calls on the app. Imagine the potential for video ads customers watch while on hold for a support rep.

Video is an area where new revenue could come first. Facebook reached more than 4 billion daily video views last quarter, with 75 percent of them on mobile devices. In the earnings call, Sheryl Sandberg cautioned that “it's still early days and we're still focused on quality” and that video ads would displace existing newsfeed ads (albeit at likely higher ad rates). So any revenue from video ads could be slow to appear in 2015.

In short, Facebook is asking investors to bear with it while it carefully cultivates new areas of business. Again, those who heeded that plea two years ago were rewarded. The difference this time is that Facebook isn't undervalued – it's trading above 40 times its estimated earnings this year – while the risk of something going wrong hasn't diminshed.

In that sense, Facebook remains a fairly safe bet. But it's an expensive one. And by no means sure.