Your Privacy Kind of Sucks, Fix it Up This Weekend
Since it's hard to trust what companies that get your data will do with it, the best way to maintain your privacy online is to minimize what you give them. Create accounts with pseudonyms or variants of your name (yes, even Facebook and Google+). Supply only profile information that is absolutely necessary, and lie about critical stuff like birthdates. Turn off all app- and ad-related functionality on Facebook and other sites wherever possible. Aggressively manage all forms of cookies and other persistent storage. Actively block ads and other trackers. Diversify your digital life by using services from different companies, and provide each with different profile information and email addresses. And remember that "opt out" usually does not mean your information won't be collected and stored, it generally only means that you won't be served up constant reminders of it (e.g., targeted ads, browsable web history, etc).
Related: Security - Privacy - Back Ups - Hacking - Downloading