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At a guess; dropbox lets you share public files; both privately syncing them with others or presenting them publicly.

All of which is non-indexed and not easily tracked.

Im guessing it's become a distribution method for information they are not keen on (or they feel it will be).

In a sense it was only a matter of time :(

Do they block P2P as well? I was under the impression piracy through bittorrent was pretty rampant.

I guess it is blocked because Dropbox was used to share software like GAppProxy/MyEntunnel/Puff, which can be used to surf across the GFW.