1. Kids under 13 need time to be alone, to think and dream and play. Facebook robs them of this time.
2. Kids under 13 do not comprehend how Facebook can destroy their privacy. Years later, when they apply for their first job, they may be turned down because of an idiotic comment or a stupid picture they posted on Facebook.
3. Every hour on Facebook is an hour stolen from what could have been a priceless education. A single example: If your kids under 13 spend even one hour a day playing with children who speak another language, they will grow up bilingual in some language such as Russian, Arabic or Mandarin Chinese. Which would make you prouder? The dual language skills, or that they were hopelessly addicted to Facebook?