Mig33 is most downloaded mobile app — have you heard of it?

Mig33, a company that lets millions of users communicate on mobile phones via IM,  chat rooms, and photo sharing, says it’s the most popular downloaded mobile application ever.

You’d hardly know it, though. Chances are, if you live in the US, even if you’re tech-savvy, you’ve never heard of Mig33.

Sounds like the Russian fighter jet, right?

Most of its users are on the hundreds of millions of low-end phones sold by Nokia and other Java-based phones that have most of their distribution in developing countries. Mig33 is increasingly popular on Windows-based phones, and Blackberries, but is not on the iPhone, and has yet to make a real dent in the U.S.

Still, it says it has been downloaded more than 50 million times and has 20 million “registered” users, or those who have actually signed up for membership for the service. The company does not reveal how many “active” users it has, i.e, the number who have used the site within the last 30 days. Steve Boom, the company’s chief executive, says that number is in the “millions” and that from his best estimates, that make Mig33 “the biggest” app period.

On the iPhone, exact numbers of downloads aren’t released by Apple, but from available data from Apple and Comscore, Boom estimates the top apps (game like Tap Tap Revenge and social networking app Facebook, and Google Earth and music company Pandora) have about 11 million downloads or so.

Now here’s a dirty little secret about Mig33’s success: The company pays Getjar, a company that runs a mobile application store, to market the Mig33 application on its portal. It does so through a “Pay Per Download” program, whereby Mig33 bids in an auction for favorable placement on Getjar’s screen: Users who come to Getjar to peruse cool apps see Mig33’s app featured, and then click to download it. Getjar once accounted for half of Mig33’s overall downloads, though that number is down to about 20 percent recently — after Mig33 expanded its other marketing efforts.

Mig33’s Boom won’t say how much he has spent overall on the Getjar campaign. However, Getjar’s chef executive Ilja Laurs tells me companies bid in a range between 1 and 50 cents per download, and the average bid costs about 5 cents in the developing world and 20 cents in the developed world. That means Mig33 spent somewhere south of $1 million for its more than 10 million downloads from Getjar, based on my back-of-the-envelope math.

To be fair to Mig33, it doesn’t rely solely on traffic from paying Getjar. The overwhelming majority of its users have come virally — from users telling other users about the service, according to Boom. Although the company’s based in Burlingame, Calif., its main base of users is in Asia. It has expanded its offerings lately, including selling virtual goods.

(Note: Getjar devotes about 20 percent of its screen space to feature pay-per-download applications, of which Mig33 is just one. Nimbuzz and eBuddy, which offer competitive applications to Mig33, have also paid for a huge number of downloads from Getjar. Mig33 is only the second most popular application on GetJar. Opera Mini is the most popular at 19.1M downloads, and Opera hasn’t paid for those downloads. Also, Getjar’s traffic is actually growing at a faster rate than ever, despite the emerging competition from the iPhone: In May, Getjar had 32 million downloads a month, up from 20 million in January, says CEO Laurs. It has 15 million unique users. The question is, how long will that leadership position last? Already, Apple has sold 40 million iPhones and iPod-Touches globally, and so visitors to Apple’s App Store may already outnumber visitors to Getjar. We don’t know for certain, because Apple doesn’t release such figures.)

Mig33 was a winner of our Tesla award for top 10 best mobile startups at our MobileBeat conference last year. The company has raised more than $23.5 million in two rounds of funding from Accel Partners, DCM, Redpoint Partners and TVP.

[Are you an entrepreneur or executive active in mobile? Join us at MobileBeat 2009, our mobile conference for industry leaders, where folks like Getjar’s Ilja Laurs and other app store leaders, such as Nokia’s Tero Ojanpera and Google Android’s Eric Chu will be speaking. Sign up soon.]