Amongst layman's or techies (coders, etc.) Most "normal" people don't seem to know what DropBox is outside of "introduction" via a "techie." Very anecdotal..it would be interesting to note paying vs free users, which could be (very lightly) related to layman / techie usage (VERY lightly).
September 2008: 100,000 registered users
January 2010 (15 mos later): 4,000,000
Mostly from word-of-mouth and viral:
35% of daily signups from referral program
20% from shared folders, other viral features
Sustained 15-20%+ month-over-month growth since launch"