Whenever we post on Facebook, twitter, or instagram, we leave traces of our lives and actions. Police can gain access to your social media sites and use it against you in court.
Facebook comes out with a new and easy to read guide on what information they have access to, and actually backup. Information such as what you search, what you say, and your aliases are, are all being kept. This information is considered, "non-personally identifiable information" and can be useful for marketing, the government, and anyone else who might want to dig up information.