The problem with facebook apps is that your friends apps can get YOUR info unless you have your privacy settings setup correctly.
What your friends can share about you through applications and websites
When your friend visits a Facebook-enhanced application or website, they may want to share certain information to make the experience more social. For example, a greetings card application may use your birthday information to prompt your friend to send a card.
If your friend uses an application that you do not use, you can control what types of information the application can access. Please note that applications will always be able to access your publicly available information (name, Profile picture, gender, current city, networks, friend list and pages) and information that is visible to everyone.