Dropbox inventor determined to build the next Apple or Google Four years ago, Drew Houston was just another super-smart hacker with ambitions of starting his own company. He'd strap on headphones to block out everything but the endorphin rush as he cranked code late into the night on a new service that instantly syncs all of your files on all of your devices. Houston, who played guitar in a '90s rock cover band at Boston bars and college parties, dubbed it "Even Flow" after one of his favorite Pearl Jam songs. On a white board in his Cambridge, Mass., apartment, he calculated that he'd need several hundred users to "not feel like an idiot" quitting his $85,000-a-year job as a software engineer. Today, Houston needs software to track how many people use his service. Dropbox has more than 50 million users and adds another every second. It's one of the fastest-growing companies Silicon Valley has ever seen. Both Apple's Steve Jobs and Google's Sergey Brin sounded out Houston about buying Dropbox. But Houston says he's determined to build the next Apple or Google, not sell out to them. And some of high-tech's smartest money is backing Houston's vision. Dropbox has figured out an elegant solution to a vexing problem. With the explosion of smartphones and tablets, people have more devices and more apps than ever before. How can they get access to the latest version of all their stuff - photos, music, videos, documents, spreadsheets - no matter what device they are using and no matter where they are? For millions, the answer has been Dropbox. Every day, 325 million files are saved on the service. Dropbox has become a verb as in "Dropbox me." In September, Houston pocketed $250 million from seven of Silicon Valley's top venture capital firms. That eye-popping sum pegged the value of his company at $4 billion and his own net worth - at least on paper - at an estimated $600 million. Now this 28-year-old chief executive has to make sure Dropbox becomes the next Facebook, not the next MySpace. Dropbox faces potentially lethal competition from some of the world's largest tech companies and dozens of start-ups packing piles of cash and top engineers. It may have won over consumers for now (without spending a cent on marketing, just giving away free storage as an incentive for people to tell their friends). Homeowners who are remodeling use the service to pore over contracts and tweak design plans from architects and contractors. Couples planning weddings swap drafts of invitations and wedding cake photos. Astronomers upload and share giant telescopic images of the heavens taken all over the globe. Walter Isaacson even used Dropbox for his bestselling biography, "Steve Jobs," even though Apple runs a competing service. But consumers can be fickle. What will happen when Apple, Google and Microsoft point their big guns at Dropbox in the fight to become the spot that houses everyone's digital stuff? "It is clear that Dropbox is going to have serious competition not just from the Apples of the world but everyone else," said Tim Bajarin, president of technology consulting firm Creative Strategies. It needs to innovate to stay ahead of the pack. Houston knows Dropbox can't afford to coast. He says he wants Dropbox to find its way onto every device you use, be it your smartphone, camera, TV remote, even your car, and to become the way you collaborate on files, listen to music or share photos. People may know us today as the magic folder on their desktop or the app on their phone. But we see ourselves as building the Internet's file system," he said. One reason people are betting on Dropbox is that the technorati have so eagerly embraced it, reminiscent of the cult-like enthusiasm for Apple products. And they have embraced Houston, who these days is the tech world's equivalent of a rock star. Strangers stop him on the street and in Starbucks. The guy who used to grub on Hot Pockets in his lean start-up days has rubbed elbows with wealthy donors at a $38,500-a-plate dinner for President Obama, and he and his tech pals broke bread with Gov. Jerry Brown. And Houston got to meet Pearl Jam lead singer Eddie Vedder backstage at a benefit concert last year. Houston, who has spiky Elvis Costello hair and rarely deviates from his uniform of jeans and a Dropbox hoodie, says he gets his biggest rush from peeking over someone's shoulder in a coffee shop and spotting his company's logo on their laptop screen. At Dropbox's San Francisco headquarters, Houston (pronounced like the Manhattan street, not the city in Texas) sits in a sea of engineers under a neon sign that reads "ITJUSTWORKS" with "just work" flashing in blue. His desk is blanketed in a flurry of resumes an inch think and his attention constantly darts between four 24-inch monitors.