A lot of seemingly innocent personal information on social media sites can be useful to identity thieves. Home addresses, phone numbers, and birth dates are all examples. Identity thieves can use your pictures to fake an ID card.
Even information or pictures of your family can be used by identity thieves: Remember, family-related information, like "What is your mother's maiden name?" is often used as security questions for your bank or other personal accounts. Even announcing your whereabouts or vacation on a social media site can present a threat: a potential thief will know that you are not home.
For these reasons, it is very important to be careful of what you post, and to review your privacy settings on any given social media site to make sure your information cannot be viewed by strangers.
For a guide to Facebook privacy settings, see Facebook family privacy: What are the 5 major settings to look at?