balloonsToday, three and a half years after mobile ad network AdMob was founded by serial entrepreneur Omar Hamoui, the company is about to serve its 100 billionth ad. That’s 14 ad impressions for everyone on Earth. AdMob claims, probably rightly, to be the world’s largest mobile ad server, even ahead of Millennial Media, which serves the majority of the ads delivered to America’s most-subscribed wireless carrier, Verizon.

AdMob’s milestone comes just as the Great Advertising Reset of 2008-2009 seems to have ended. Ad spending in the past three months, our industry sources tell us, has been rising again. Yes, we’ve thrown away our crazy forecasts from early 2008 that claimed “AdMob is about to mint money.” But looking forward, we see our new, more sensible predictions for growth being fulfilled right now, something we think analysts’ reports will confirm in coming weeks.admob2

Here’s what you need to know about the new playing field: It’s shaken out as not one, but two very different markets.

1. The most money comes from a few top advertisers on a few top sites — The ad market doesn’t follow the Internet-fashionable Long Tail theory, which says there’s more money to be made by serving millions of small advertisers to millions of small sites, instead of landing the top few accounts. AdMob and Millennial agree with Millennial’s CEO’s claim that “the bulk of the reach in mobile is in the largest 100 sites versus the long tail of the wired Web.” That’s why AdMob has deals with ESPN, CBS, Geico and Starbucks.

2. Ad revenues will rise, but per-ad revenue will continue to drop — This trend hasn’t changed since VentureBeat plotted it last year. The market’s growth in number of ads will continue to outstrip its growth in number of dollars spent. That means the best ways to grow will be to focus on landing million-dollar deals, while making the self-serve business for little guys more and more efficient, to keep it profitable despite the continually lowering rates they’ll pay. Of course, increased efficiency drives those rates down even further. If that excites you instead of giving you a stomachache, you may do well here.