Many of Google's apps have passed a billion installs. This sounds like a large accomplishment, until you consider that shipping pre-installed counts as being installed. In that case, it's more impressive when third-party software makes the list. Dropbox isn't at a billion yet, but it's now halfway there.
You can probably say you know more people storing stuff in Dropbox than Drive, but the latter Android app hit a billion quite a while ago (again, I wonder why). Now, it's worth pointing out that Dropbox is not an uncommon sight among apps that ship preloaded. That surely helped it get where it is.
But hey, that's enough crapping on Dropbox's accomplishment. Half a billion downloads is nice. It's nice, Dropbox. You did well.