A 5 billion valuation implies earnings of at least 250 million/year. Assuming paying customers are worth 240/year and represent 5% of the user base, dropbox would need roughly 21 million users. Maybe 5% is high; if 4% is better then the number jumps to 26 million, etc...
There may be billions of people on earth, but how many are sophisticated enough to really need file sharing? Also, there are other players such as box that will claim some of those users.
Dropbox is awesome, but I can't understand the numbers on this one.