Interesting thing about Dropbox is that, owing to their reliance on S3, you estimate pretty accurately what they spend on their users. People who go to extreme lengths to max out their referrals, etc. are outliers. I'd guesstimate the average free account consumes 1gb of overall storage, which costs Dropbox a whopping $0.06 a month. Even those maxed out, 16gb, pay-nothing accounts are charged at $0.80/month. When you consider that a 50gb account goes for $9.99 and costs them $2.75 in raw storage fees (probably less since who uses their full allotment?) you can see how easy it is for them to make money: one paid account subsidizes hundreds of free accounts. Granted this ignores transfer and request costs, so the margins are lower, but even with those, I'd guess they need a ridiculously low conversion rate to turn a profit.