Bottom line up front: I don't think children belong on Facebook.
Yes, Facebook is a fun place to connect, but even adults have been victimized via Facebook. Older kids, even college kids, have been bullied on Facebook. Some people think that kids can be on Facebook if properly monitored, but I disagree. Facebook has privacy issues, and kids just don't have the maturity and experience to deal with some of the issues that may be "faced" on Facebook. Facebook isn't just about playing games and posting pictures for your friends and family . . . There are people who look for potential victims on Facebook--they look for children, women, etc.
I am very careful with my own Facebook account--I wouldn't dream of allowing an elementary aged child to be on Facebook. Parents, grandparents, guardians . . . please think twice before allowing an elementary or middle school aged child to have an account on Facebook. Be a responsible adult and draw the line. I did.