Looking at the reviews and articles out there about these sites, most organizers and bloggers do not recommend these sites or social media in general for children under 13. That being said, you are the parent and you get to decide what your child uses. My personal note is that you need to monitor and discuss expectations with your child. Also realize that you might trust your child, but you need to think about what their friends will be posting and talking about as well.
Should I allow my child to have a Facebook account? Here are some ideas on the topic. . . Note: Facebook does require, even though many ignore, that you be at least 13 years old to have an account. Common Sense Media shares a review of the site —http://www.commonsensemedia.org/website-reviews/facebook Safety Web shares ideas of how to allow your child to use Facebook and try to keep them safe —http://www.safetyweb.com/facebook-monitoring-for-kids-teens-safety
Instagram is an app to share pictures and videos. It has settings that allow the account to be private but otherwise the pictures and videos that children take are public. Common Sense Media review http://www.commonsensemedia.org/mobile-app-reviews/instagram/user-reviews Your Sphere review http://internet-safety.yoursphere.com/2012/02/instagram-is-it-okay-for-kids-what-parents-need-to-know/
Kik and Snap Chat are two social messaging apps. Snap Chat