If Facebook is ever to pull off an IPO it needs to show investors it hasn’t yet hit a wall in terms of user growth. Even with the magic 500 million users announcement looming, Facebook founder Mark Zuckerberg is getting a bit anxious about where to find the next 500 million. The answer, Zuckerberg told reporters earlier this week, is: Japan, Russia, South Korea and China where local social networks are eating Facebook’s lunch. We would also add Brazil and India to that list. Can Facebook achieve global dominance? It is already the dominant social networking force in the English-speaking world and across much of Western Europe, but sites like RenRen in China, Orkut in Brazil and India and Vkontakte in the former Soviet bloc are the dominant players there. Facebook has impressively added 70 translated versions of the site in the past two years, helping it to expand at a clip of 3 percent each week. Ah, the good old days.

“We saw our exponential growth rate continue for a very long period of time, and it still does at a super-linear rate, though not quite 3 per cent a week any more,” Zuckerberg told Inside Facebook.

In order to retain that phenomenal growth rate, Facebook has the BRICK (Brazil, Russia, India, China and South Korea) countries plus Japan in its sites. Here’s the steep challenges it faces in each region (click to enlarge):