10x more devices. Paradoxically, there's less room for ads.

Mary Meeker was one of the first Wall Street analysts to realize the potential of the internet. In the 1990s she rode the first tech bubble to the top, successfully predicting the rise of Amazon, AOL and Netscape, and then got beaten up by the press when that bubble burst and other stock picks failed.

Now Meeker is back with a glowing forecast of economic recovery and mobile adoption. The most interesting slide for marketers may be No. 32, which points to a continued rapidly fragmenting media world of 10 times more devices in consumers' hands. In 2000 there were about 1 billion computers or cell phones on our planet; by 2010 the device plethora is expected to be 10 billion units. The only possible behavior that can support such expansion is concurrent media usage, a trend observed by Nielsen in which consumers use television, smartphones, Kindles, tablets, games and GPS systems all at the same time. Do you have teenage kids? Check what's in their hands when they're watching the tube.

A casual viewer of Meeker's rosy outlook might think mobile phones point to huge opportunities for advertisers (yes, a new platform to push out our message!); a more critical observer will realize that in a world filled with devices for sharing information across multiple channels simultaneously, the ability of consumers to focus on any single ad message is going to be diminished. (Back to those teens: where do their eyes go when commercials come on TV? Why, down to the laptop in their laps.) Play the device expansion forward to 2020, when chips and tiny glass screens and video walls are everywhere, and something has to give. The size of small screens on smartphones leaves about 90% less visual space for ads vs. a computer screen. A simple ruler shows you the severe contraction in advertising inventory approaching ... in the very hyped-up devices, cell phones, that are the future of consumer communication.

The paradox: More ad channels, less real ad inventory

What's that? Ad inventory is shrinking? This seems nonsensical, since we're approaching 10 billion devices all filled with colorful screens. But it's true. While media buyers can find low CPMs and millions of slots for banners or mobile apps, the real impressions on the audience are growing diminished. In terms of consumers actually seeing the ads -- you know, eye contact required to make the message sink into the brain -- tiny screens, concurrent media usage, and shifts in consumer modality from watching to creating all squeeze the "real" ad inventory making impressions on your target customers.

It's a classic supply and demand problem. If demand for third-party interruptions is falling as consumers learn to make their own content (witness Hulu.com's struggle to make money with limited consumer patience for online video ads), and the supply of interruptions grows higher (as advertisers try to squeeze into every emerging channel), the value of those interruptions will fall. Substitute the word "advertising" for "interruption," and you'll see marketers' challenge.