You're not alone. I think this is a serious problem for Dropbox. When people reach their quota they have the following options: 1. Pay up and upgrade; 2. Spam friends to get more credits; 3. Sign up for another service, FOR FREE. We all know what's the easiest to do.
This is but one data-point, but perhaps 3rd party app storage and general file storage are features that don't complement each other at all.