Facebook has been accused of not being safe and to reduce individual privacy. But if you are aware of the risks when you use facebook you do not ned worry.
If you use the same password on multiple websites and one of the sites would be hacked (ie that any unauthorized person will get over your account details) or if they come out in another way, it can rapidly spread to rogue individuals. They can then use this information to get into other websites where you have used the same user account data.
Use different passwords on different sites and do not use passwords that are too easy to guess at. Ideally at least 8 characters where you mix large and small passwords, numbers and other characters.
I know it is tough, but it may be worthwhile.
There are already examples of hackers who come over account information from facebook. The risk is not imminent that it will happen to you but it is safe to assume it. Do not use facebook for stuff that you absolutely want to keep as a secret.
You can not be absolutely sure that everything you do on facebook remains a secret. Images for example is easy to find by searchin on Google and there is nothing to prevent a friend to spread images or the stuff you write.