43,570,999 user accounts were breached in a hack of Last.fm that occurred in March of 2012, according to a report from LeakedSource.
The number of passwords and the severity of the hack were not uncovered until today. The passwords were stored using unsalted MD5 hashing. Rather than storing passwords in plaintext, nearly every site that stores critical user information utilizes some form of hashing. Hashing is a method for encrypting data, but some methods are far superior to others.