The weakest link of security of internet services are the users themselves. 47% users use the same login for other services.
The survey also proved that if the users are allowed to choose their own login, in 65% cases they choose ones which they are already using for other services, such as social networks or e-mail. These may not be secure enough.
This fact is the target of hackers. If they manage to steal access information for one of the services, they will try to use this information for logging in other accounts of the same user. This way all accounts with the same password are at risk.
Repeated use of passwords also threatens the security of companies. These often let employees choose passwords for access to corporate computers and servers. Many of them choose simple and repeatedly used password in order to remember them easily. It is a matter of a few minutes to find where the user works thanks to social networks and other such services. Eventual way for the attempt to attack is, thus, open.
(source http://www.net-security.org/)