Anecdotally, I see FolderShare quite widely used. I have no idea how number of users compares to DropBox though.

I think what he's getting at is that if Microsoft has a similar product and they gained 2 million users, it would not be seen as impressive, because that is still far less than 1% of all Windows users.

So, while Microsoft may have a competing product with many millions of users, at their scale it doesn't get the same recognition as a startup gaining 2 million users.