I have no idea how to pin a number like $10B or any other on DropBox. I will say this:
DropBox has potential to be much more than about syncing and sharing files in the cloud. If they manage to come up with a useful enough "app API", they could grow into the backbone for the myriad SaaS apps where people worry "what happens to my data when 'tiny SaaS start-up' dries up and blows away?" or maybe more appropriately "who owns my data and how do I ensure it's secure, backed up, and accessible through 'neutral, well understood, and reliable means'?".
DropBox has potential to be much more than about syncing and sharing files in the cloud. If they manage to come up with a useful enough "app API", they could grow into the backbone for the myriad SaaS apps where people worry "what happens to my data when 'tiny SaaS start-up' dries up and blows away?" or maybe more appropriately "who owns my data and how do I ensure it's secure, backed up, and accessible through 'neutral, well understood, and reliable means'?".