Facebook is a business. It exists to take your online activity and turn it into revenue. Facebook is free. But there is a cost. You're paying by being exposed to advertising and allowing limited disclosure of your online activity.
How limited is the disclosure?
Basically every-thing you post, every person you befriend, every group you join will be made public to your "friends", "friends of your friends" or "everyone" - depending on your privacy settings.
Ask yourself – who would I like to see what is on my Facebook
Many employers check your Facebook
People have lost their jobs and have had lawsuits against them because of status updates
Use a unique and strong password
Don’t let your internet browser remember your password
Lock your PC when you step away from it
Filter your friends
Restrict what different friends/acquaintances/family/colleagues/employer can see
Use professional networks for employment (e.g., linkedIn)
Be careful what you "like" – some pages are scams
Don't rely on Facebook to protect your privacy
Ensure your PC has virus software to protect your password
Use Facebook's one true security feature – it will inform you anytime any new device accesses your account. That means if some PC or smartphone you've never used before logs into your account, Facebook will email you. To turn this feature on, go to "Account Settings". Then select "Account Security".
Adjust your privacy settings, don't end up on Ellen!
A short clip on how to adjust your privacy settings