And don't forget companies of all sizes are going into cloud: so replace 21 million users with 21 millin users via their employeer. (Ever tried sharing documents using MS SharePoint?)
In short, I think Dropbox got on to something big early on, but they will eventually be like the early Netscape of this area.... Other competitors are pretty much guaranteed to come in and erode their value with "free" (enough) alternatives that exist to prop up other business models.
Early on? Just the opposite. The space was so saturated that new entries into it were considered a cliche joke. http://www.joelonsoftware.com/items/2008/05/01.html
The other great example of successful late entry into a saturated space, of course, is Google.
Claiming the existence of those older services diminishes the value of Dropbox (or current Live Mesh, etc) is like saying that the iPad is no big deal because Fujitsu was making tablet computers almost 20 years ago.
Dropbox's simplicity is it's own undoing in the long run; it's too easy to commoditize. They'll be forced to lower their price to the cost of storage and that's when Apple and Microsoft will just subsidize their own offerings and kill them, unless a regulator notices it on time.
Google is not a good counter example; it seems simple to the user, but as Bing has shown, it's very hard to replicate. Dropbox on the other hand is trivial to clone. Any IPO event would just increase the likeliness of big companies noticing and putting some effort into it.
However, it might be that as cloud services become a critical for businesses and individuals will not be going for "free" solutions. Reminds of "Starbucks dilema": wanna a cup of coffee for 50c and have diarrhea or Starbucks coffee for $4? And how hard could be to make a good coffee?