That's a major error on their part. Surely, errors like that can ruin a business such as theirs. In addition, it doesn't do the reputation of other cloud-computing-based systems any good either.
I have never had great faith in cloud-based computing, and errors such as this only go to strenthen my views. We're at the mercy of their security, and any breach results in more data being lost than if it were a local data breach.
I use dropbox for temporary file sharing of large files. I usually remove the files within 24 hours, and certainly as soon as the person i'm sharing the file with has downloaded it.
I never use it for sensitive data.
The only way to be sure you don't get hacked is to physically disconnect from the Internet.
