While the glitz, glamour, and buzz of Silicon Valley tends to make start-ups seem as though they can grow effortlessly like weeds, few tech start-ups have grown quite as fast as Dropbox. Since January 2010, the file-syncing and sharing service grew from 4 million users to a staggering 25 million users. There are now more than 200 million files saved every day. And, while these numbers would be impressive even with the help of a snazzy ad agency, Dropbox has spent absolutely no money on advertising. Dropbox’s rapid growth can be attributed to the fierce loyalty of its users. The service allows…