They do have something like 40 employees, so they're spending on the order of $10M/year on salaries + benefits + office + etc.
That said, I get the impression (from how PG talks about them, the fact that they seem to have supported this massive growth in users & employees without more funding, etc) that they are doing very well, and improving quickly.
I don't know - the Backblaze guys did several blog posts on the economics of online file hosting/backups and it came out very expensive in terms of hardware. Plus the custom software and management. Dropbox accounts are generally small, so maybe that's making it easier. I'm not saying they aren't doing fine, just that online backups are low margin (Dropbox is special, they aren't regular 'online backup' providers, I realize; and they operate at a premium price point; but still...
The backup services are a different beast. Huge competition, AdWords cost is $5 per click minimum, frequently as high as $15-20. This translates into an insane customer acquisition cost, $50+ is not unheard of, meaning in turn they need people stay with them for 2-3 years just to turn some profit.
The online backup looks simple, and technically it is. It's the business side that is tough.
That said, I get the impression (from how PG talks about them, the fact that they seem to have supported this massive growth in users & employees without more funding, etc) that they are doing very well, and improving quickly.