You are able to choose who gets to see your content.
Notes, Photos, Groups, Wall, etc. You can set who gets to see them.
The three settings are Everyone, Friends, and Private.
If you navigate to Account > Privacy you can change the visibility of your activity wall
Here you can also control if people can see you online, who can send you messages, and who can see your bookmarks.
When you go to create a photo album, post a note, or create a group you will find you can control privacy there as well
Those directions will be in the corresponding section