Facebook’s user base is growing much faster outside of the U.S. than it is in its home market, but it has a little problem: those users are continuing to generate far less revenue than U.S. consumers do for the company. Facebook noted in its Q4 earnings today that average revneues per user (ARPU) for the U.S. and Canada in the quarter was $4.08, compared to $1.71 in Europe, $0.69 in Asia and only $0.56 in the rest of the world. That makes for a disproportionate amount coming from the U.S:

The same goes for advertising revenues per users: in the U.S. users are generating $639 in advertising revenues, with about half as much, $374, coming from Europe, with $168 in Asia and $156 in the rest of the world.

Overall the U.S. and Canada accounted for about half of all of Facebook’s revenues, at $2.5 billion out of $5 billion, although it is only about 18% of the total user base.

The issue for Facebook is that its newer users are in countries that are less wealthy than the U.S., and many of them are accessing the service on devices that may not be able to receive high quality ads (ie feature phones rather than smartphones or PCs), and they will tend to spend less money on services like buying credits for Facebook games. And these users almost surely won’t have the money to spend $15 on a Facebook Gift, which raises serious questions about how this will scale longer term.

“We’re getting into some of the lower monetizing countries in Europe, they’re starting at a lower price point and bringing down the average,” Facebook’s CFO David Ebersman explained on the conference call. Later he noted that, in the context of games and gaming revenues, the U.S. and other mature markets are stalling in their growth. “We’re not growing the essence of this user base in the developed markets.”

Taking Facebook’s total monthly active user base of 1.056 billion users, in Q4, there were 193 million monthly active users in the U.S. about 18% of Facebook’s total user base.

That’s an increase of only 4 million on the previous quarter, and only 7.8% on the same quarter a year ago. The growth rates are much higher outside the U.S.: Europe had MAUs of 261 million, a rise of 8 million, and a rise of 10.5% on last year. Asia grew by 21 million users to 298 million over Q3. And the rest of the world grew by 40.6% on last year. Rest of the world was up by 16 million on Q3 to 304 million, and 35% on last year.

As the developing world transitions to smartphones the question is whether Facebook will be able to ride that wave and get their newest members to change their behavior and become more valuable for the company.