Facebook (FB) CEO Mark Zuckerberg started his company’s second quarter earnings announcement with what might have been the understatement of the year: “We had a good quarter.”

Indeed, Facebook earnings knocked the cover off the ball. Earnings came in at 42 cents per share vs. the consensus estimate of 32 cents, and revenues came in at $2.9 billion vs. the consensus estimate of $2.8 billion.

By any measure you choose — earnings, revenues, margins, user growth, average revenues per user — Facebook had a solid quarter of growth.

Per the earnings release:

• Daily active users increased 19% year-over-year to 829 million.
• Monthly active users increased 14% year-over-year to 1.3 billion
• Revenue for the second quarter increased by 61% year-over-year and revenue from advertising increased by 67%.
• Mobile advertising revenue increased to 62% total revenues, up from 41% in the second quarter of 2013.

And best of all, margins improved: GAAP operating margin rose to 48% in the second quarter of 2014 — up from 31% in the second quarter of 2013.

Digging a little deeper, almost all of the user growth came from outside the U.S. and Europe. FB added about 5 million new users in the developed world but 36 million from Asia and the rest of the world. And Facebook is doing a better job of monetizing these emerging market users: About 38% of the revenue growth over last quarter is from Asia and rest of the world.

North American users are still by far the most lucrative in terms of revenue per user, generating more than double the revenue of European users and almost six times the revenue of Asian users. And this gap isn’t really closing just yet; in fact, it’s getting wider.

This is a problem as well as an opportunity. Facebook is doing a great job of monetizing its North American users, but this demographic is now saturated. Facebook’s user growth is almost entirely in Asia, Latin America and other emerging markets, where revenues per user are much lower.

So, for Facebook earnings to continue delivering the growth that investors have come to expect, one of two things have to happen: Either Facebook continues to squeeze more and more revenue out of its North American users — which would seem a little unrealistic to me with their current advertising model — or they need to better monetize their emerging-market customers.

It could happen. But valued at 22 time sales and 56 times trailing earnings, it would seem that torrid growth is already priced in. If Facebook hits any bumps in the road, expect the stock price to take a  tumble.

Meanwhile, I expect the growth in new users to start tapering off soon. At 1.3 billion — fully 18% of humanity — is already a monthly active user. But only about 40% of the world’s population currently has access to the internet; this means that roughly half of all people with internet access are already regular users.

And Facebook already accounts for about 20% of all time spent on the internet; it’s hard to see that number drifting much higher.

In the CNBC interview, I said you could think of Facebook as a bet on the horse (Facebook’s advertising model) or a bet on the jockey (Mark Zuckerberg). At current prices, I would argue that you’re essentially betting on Zuckerberg’s ability to find new income streams.

Will he? Maybe. He certainly has a good track record. But before you buy Facebook stock, make sure that’s a bet you’re comfortable making.

Charles Lewis Sizemore, CFA, is the editor of Macro Trend Investor and chief investment officer of the investment firm Sizemore Capital Management. As of this writing, he did not hold a position in any of the aforementioned securities. Click here to receive his FREE weekly e-letter covering top market insights, trends, and the best stocks and ETFs to profit from today’s best global value plays.