It’s been revealed that Facebook has stored millions of user login passwords in plain text so employees could read them. It’s now fixed but they haven’t reset passwords for those affected. If you use the same password on multiple sites (you really should change that habit) then you definitely should change it ASAP both on Facebook and any other sites where you’ve used the same password.
You can read all about it here at wired.com.
While talking about Facebook security, it is very wise to implement their two-step authentication. This is where the password isn’t enough to log in, you also need to enter a code which is accessed on your mobile and changes every 30 seconds. If you’ve not activated that, please consider doing so.