But with each incremental user, Dropbox's value increases in an e^x fashion.
For more on why I think so, here is a more detailed breakdown: http://marcgayle.com/how-dropbox-is-printing-money
There is no reason that Dropbox's growth will slow any time soon. It is a fabulous product, and has the referral system built-in.
It's the closest elixir to magic I have seen in recent times.