According to a Privacy Impact Statement released this month from Homeland Security, they monitor several social media sites for specific keywords. It seems obvious that what you post to the public online is public. However, users tend to cry foul when someone starts tracking them across multiple networks and compiling their data. Homeland Security is doing just that if you happen to make a public facebook comment with a keyword on their list, such as Yuma, Subway, Smart, Ice, or China. Once your status update is picked up, it becomes a record of the agency and is stored accordingly.
Interestingly, President Obama believes Islam is a religion of peace, meanwhile, he wants to know when anyone tweets Jihad or Islamist.
Be sure to jump to the end and read Appendix A and B of the Privacy Impact Statement.