Typical conversion rate is 1-2%. That's at least 40,000 paying customers. At $10/mo (that's the maximum revenue per subscriber) that's $400,000 a month, or $4.8M a year. The key here is they are a growing, revenue-generating company.

Considering that Facebook has like 500m users, yet doesn't charge any of them, and they're valued at $50B or whatever, and yet DropBox has 4m users and does bring in $10+/month from a good percentage of them, I'm going to guestimate that DropBox is profitable in at least the millions of dollars annually range. And that's a very conservative guestimate.