"Single sign-on (SSO) makes life easier and more secure for everyone." (https://www.dropbox.com/help/)
"7 million Dropbox username/password pairs apparently leaked."
"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 been expired as well."