Twitter’s user base is growing like gangbusters in the Asia-Pacific region and other areas outside the U.S., but three-quarters of company’s revenue comes from users in the U.S.

That’s the key dilemma painted by eMarketer’s first-ever forecast of worldwide Twitter users, released today.

By contrast, Facebook is also experiencing faster user growth in other countries than in its homebase of the U.S., but, according to eMarketer PR director Daniel Marcec, “53 percent of its revenue in 2013 came from international markets.”

“The two companies are at different points in their growth trajectories,” he pointed out — that is, at different points in their life cycles. Twitter was founded two years after Facebook.

Twitter has said that, with its revenue-per-timeline metric, it brings in $3.47 per 1000 timeline views in the U.S., but only 61 cents per 1000 elsewhere. Facebook’s non-U.S. users similarly generate less revenue per user than U.S. users, but the differences aren’t as great as Twitter’s — and Facebook already is getting the bulk of its revenue from elsewhere.

Twitter is looking for a big boost in user growth, so it can attract more ads and higher rates. In the most recently completed quarter, Twitter showed a growth rate of 25 percent, down from 30 percent in the first quarter, and eMarketer is now forecasting 24.4 percent for this year. But that’s not the overall growth rate that Wall Street wants to see.

Nearly a third of all Twitterers will reside in the Asia-Pacific region this year, compared to less than a quarter in the next largest region, North America. And in five years, the Asia Pacific will hold more than 40 percent.

By the way, these figures don’t yet include China, which officially bans the service.

Twitter user growth

India and Indonesia, which already have the third- and fourth-largest Twitter populations, will see growth rates over 50 percent. Marcec pointed to India “as a great example” of potential growth for Twitter in an emerging market, with “a young population … of highly mobile users.”

Back in the U.S., which is just over 20 percent of the total user base, eMarketer projects that Twitter’s growth rate will drop into the single digits this year.

Facebook’s user growth pattern has been similar. eMarketer projects 15.5 percent this year in North America, for instance, down from 17 and 18.8 percent in the two years previously — and dropping to 11.8 percent within five years.

Asia Pacific, on the other hand, has been adding a few points every year, reaching about a third of Facebook’s user base by 2018 – again, without counting China. India and Indonesia are similarly the top two countries with the highest growing Facebook population.