admob3Mobile ad network AdMob expects to serve its 100 billionth ad today, which averages out to 14 of ’em for every one of us on the planet. Mobile phones now outnumber PCs worldwide by 4 billion to 1.5 billion, according to the United Nations.

The mobile ad market lets almost anyone buy a couple of ads, and almost anyone serve them on their website. But the money, insiders agree, is at the premium end of the business: Big ad buys on big sites. That’s beginning to draw the attention of big companies. It’s likely AdMob will have several much larger corporations as competitors next year.

I asked Wendy Dikes, Admob’s director of product marketing, to answer my nagging questions after reading today’s news coverage.

Is AdMob profitable? Give us at least a ballpark idea of revenue.

We don’t publicly disclose revenue numbers. We did discuss the fact that we were cash-flow positive when we announced our Series C funding. [The C round announced in late 2008 and early 2009 brought in nearly $30 million from Sequoia, Draper Fisher Jurvetson, Accel and Northgate.]

Who are your top 10 advertisers? AdMob and Millennial Media execs, your closest competitors, have both stated that the money is in the top accounts, not in the long tail of small self-serving clients.

We wouldn’t be comfortable disclosing which among them is “top 10” or “top 5,” but brand and performance ad buys from top well-known advertisers is a very significant piece of our business. We work with many of the largest brand advertisers in the world, including Adidas, Comedy Central, Electronic Arts, Gap, JC Penney, Marriott, Microsoft, MTV Europe, Nokia, Paramount, P&G, Sony Pictures, Toshiba, VH1 and others.

This is the part where I let you explain how you got to 100 billion impressions served on cellphones.

The explosion of smartphone devices has helped us make mobile advertising mainstream. Historically, mobile was a prohibitively complicated place to do business. But as eyeballs shift to mobile, advertising dollars follow. We’ve seen some of the Web-based giants turn their sites to mobile. In our June Mobile Metrics Report, we showed that AdMob served ads on more than 16 million unique iPhone and iPod touch devices worldwide.

This market is too huge to ignore, and only getting bigger. Nokia has already made a couple of failed attempts to become the world’s biggest ad carrier. How do you expect to stay in business against competitors that size?

AdMob is providing tools, data, and business models to bring the industry forward. For example, AdMob was the first to create unique iPhone and rich media ad units for mobile. We feel good about our ability to capitalize on our early lead.