I'm having the same problem, I have shared folders (including files). I have cleared everything out and the space used remains the same and bears no relation to the amount of online space I am really using.
I am guessing, because Dropbox are fucking useless at technical support ("why support people when we only want their money?"), that it is because the folders are still 'shared' with me. i.e. if I picked up the original invitation I could reinstate the folders.
The upshot of this is that I am off. Bye bye Dropbox. There are plenty of other consumer level cloud space providers out there. box.com seems like a good bet.
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