actually it has built its own cloud, moving out of the Amazon cloud. From Wired
“Amazon’s cloud computing service lets anyone build and operate software without setting up their own hardware. In other words, those billions of files were stored on Amazon’s machines, rather than machines owned and operated by Dropbox.
But not anymore. Over the last two-and-a-half years, Dropbox built its own vast computer network and shifted its service onto a new breed of machines designed by its own engineers, all orchestrated by a software system built by its own programmers with a brand new programming language. Drawing on the experience of Silicon Valley veterans who erected similar technology inside Internet giants like Google and Facebook and Twitter, it has successfully moved about 90 percent of those files onto this new online empire.”