Strictly, I don't think he did; Dropbox is a big thing but it is not a "killer application" as I'd define it, ie. an application that people would buy a particular platform just to get. In the way that, say, Office is for Windows, or Halo might be for Xbox (or so I hear).

Of course, it's kind of irrelevant to Dropbox; they don't have a horse in the platform race, so they're supporting all the major ones and doing very well, but they're not a killer app for any one of them. I don't think that proves that it couldn't ever have been a killer app, but it looks unlikely now that it'll be the case.