Before your comment I don't think I'd heard of Xdrive. People know Amazon, Amazon already has a lot of users, and Amazon knows how to get people to buy things.

Odds are someone is going to have an account at Amazon rather than Dropbox, and when they go to the Cloud Drive site they see a massive button to upload files and get going immediately. As compared to Dropbox where the user has to download and install something first, and then go from there.

So yeah, I think Dropbox just lost a lot of potential users — sure, they may not have been the type of users who would pay Dropbox at first, but Amazon's service is only going to get better, and they're less likely to grow into a Dropbox paying user.