Hundreds of Dropbox usernames and passwords were posted online, and the hackers reportedly have almost 7 million other compromised accounts. The breach took place because of third-party apps which can access Dropbox, and wasn't a direct breach of Dropbox.
Dropbox denied it was hacked (via The Next Web): "Dropbox has not been hacked. These usernames and passwords were unfortunately stolen from other services and used in attempts to log in to Dropbox accounts. We'd previously detected these attacks and the vast majority of the passwords posted have been expired for some time now. All other remaining passwords have expired as well."
Despite the appeal of online storage services, users must be cautious when they grant third-party applications access to accounts. While Dropbox wasn't directly hacked, it's questionable as to what apps should be allowed access - as these third-party services are now popular targets for cyberattacks.