I first met Drew Houston four years ago, when Dropbox, the file storage and synchronization company he founded in 2007, reached 25 million users. Dropbox was in the midst of a growth spurt that has carried it to more than 400 million users, and a valuation of $10 billion in private markets. Its trademark, then as now, was its fast, simple syncing of files between computers and mobile devices. It looked like an incredible business in 2011. It looks much less so now.