Suspicious of Google Drive stat
I don't believe Google Drive is as dominant as the market share mentioned here would suggest. Don't you think a lot of those accounts are a byproduct of people having GMail accounts (even at the paying customer level)? How much people actually use Drive for serious purposes might be a very different story.
On the other hand, the fact that Drive is linked to GMail does give it an edge, particularly as the integration between the two gets better. The recent addition of a button in Gmail that will let you save a file attachment to Drive, rather than downloading it only to upload it again, has made me use my Drive account more.