Great question Andrew. That's why I think most people underestimate the power of what Dropbox has done, because most don't understand it.
Well, this is what I think happens. For some people, the majority of their files will be unique - but for many people, they won't be. The more people they get using dropbox (with the free account) is the more files they can get, which means the more things they have access to that they can charge for.
In other words, as their network of files grows and their user base grows, their costs shrink and their margins get wider - because the higher the likelihood that they are to be able to charge for files that are already there.
So to put some numbers to it.
Say when they had 100 users, 5% share the same files. That means, essentially they are charging 5 people for access to the same files. However, when they reach 10,000 users more people upload similar files I suspect that 5% increases to say 7%. That means that they can now charge 700 people for access to the same files.
Imagine when they reach 1,000,000 users and 10% have the same files - that's 100,000 users they can charge for access to the same files.
I don't know what the true percentage breakdowns are, but I suspect they will be much higher than many people expect. Double digits.
In any case, even if it is not higher than people expect, network effects work for their model - like nothing I have ever seen before. The more people join, is the more files they have which increases the % of their userbase that they can charge for. So not only is the absolute numbers of users growing, but the portion of the userbase they can charge to the same files are growing too.
It's FRIKKIN BRILLIANT!!!!!