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).

Their affiliate program apparently has helped immensely with their growth. http://www.quora.com/How-effective-is-Dropboxs-referral-prog... "As of Apr 2010, the referral program with 2 sided incentive permanently increased signups by 60%

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"

This is true of any function if you evaluate it at only 3 points.

Sigh My point was that calling their growth exponential based on three points is silly, because with only three points you can fit a wide variety of curves.