I understand why, the software works really well. But given the right amount of engineering resources, anyone could duplicate it. Nothing is keeping you on Dropbox other than the quality of the software, so if better quality software comes along, you'll switch to that and Dropbox will be a distant memory of the past.

Compare that to Facebook where you can't switch because your friends won't switch, and you only use Facebook to interact with your friends. Even if there is a better social networking site than Facebook, it won't matter because nobody will use it.