Companies are desperate to understand - and profit from - mobile users. Dave Morin already knows a thing or two.

Path founder Dave Morin and mega-diagram

FORTUNE -- Dave Morin is methodically thumbing the chain links on his silver bracelet as he parses the difficulties of designing products for mobile devices. The conversation careens from one big idea to the next until Morin hits on something profound: "With mobile, life has become the platform," he says earnestly.

Morin, 31, is the founder of Path, a social network accessible only via mobile phone. Where Facebook (FB) and Google Plus (GOOG) let users connect to thousands of acquaintances, Path users can designate a maximum of 150 friends. This restriction, so the thinking goes, leads to more intimate and genuine interactions. The app, which is available for Apple- (AAPL) and Google-powered phones, features a slick, intuitive interface that makes competitors look sclerotic.

Earlier this year Path raised a Series B round of funding valuing it at $250 million. Backers include Redpoint Ventures and Kleiner Perkins Caufield & Byers, as well as Richard Branson, Yuri Milner, and Mark Pincus.

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They may be betting on Morin as much as on Path. An early member of the Facebook team, he co-invented two signature technologies, Facebook Connect and the Facebook Platform, that helped the company surge to 900 million users. Before that, the Montana native and avid skier worked in Apple's marketing department.

Nobody knows exactly where Path will lead. At some 3 million users, it hasn't achieved the phenomenal adoption of Instagram or Pinterest. But it may not matter: Morin's company is a pioneer of mobile-first design, which, he says, is time-intensive and requires attention to detail.

The deluge of mobile users threatens to upend a wide range of businesses in Silicon Valley. Trouble is, few companies really know how to design for or generate significant revenue from such users. Facebook, for instance, stumbled badly when it released a clunky iPhone app that users groaned about until a recent update. Its $1 billion purchase of photo-sharing service Instagram was motivated in part by its need for designers and engineers skilled in creating products for phones.

Morin has become a kind of mobile guru to some of the biggest names in tech. Facebook co-founder Dustin Moskovitz is a booster. "He's always thinking through every aspect of why he's building something and what that means," says Google Ventures partner Kevin Rose of Morin's design chops. Facebook's Mark Zuckerberg is also a fan. "I'm looking forward to working more closely together," Zuckerberg tells Fortune.

This story is from the September 24, 2012 issue of Fortune.