At the moment, their business model doesn't extend much beyond brokering storage space. They are simply a middle-man between end users and Amazon S3, whose value add is some software that makes the process of storing and sharing files relatively pain-free.

They're good at what they do, and their software is nice, but in the long-run, there isn't much money in what they do.

Diversification is the obvious course of action, but it pits them against major players like Apple, Google, and Microsoft. All of these could probably afford to offer unlimited storage to all their users tomorrow, and if they got the software right, could render Dropbox redundant.