My 10-year old daughter wants to be on Facebook because all her friends are. What do you think?
Consider that Facebook (and most other social media sites) restricts membership to those who are NOT under thirteen, which answers your question on the spot. Personally, I think that if you either give your child permission to use Facebook or set up an account for her, you are sending a message that breaking the rules is okay if “all her friends are” doing it. We don’t need to extrapolate the long-term possible consequences of that message!
Let’s also consider the kind of messages that she may read or be subjected to. Although her friends may all be very nice, their friends may NOT be. At the very least, the result could be posts that you would not want your child to read; at worst, she could become a victim of cyber-bullying or decide to meet a stranger. Even if you monitor the conversations closely, the damage could be done before you can intervene.
What your daughter has done, however, is open the door for you to discuss Internet safety in a rational and safe environment. Seize the opportunity! Check out the link to the Internet safety book on the sidebar of this blog to get yourself started. Although the activity book was designed for teachers to use with younger students, it will give you a place to start!
http://www.facebook.com/terms.php
Comments:
I’m sure that lots of parents are getting asked this question. At this age, I think Facebook makes it easier by having a minimum age rule. As both you and Robin say, it can open discussions for other online activities. I bet there are some parents wishing that there cell phone companies would set a minimum age. That might help parents with the debate on when he/she is old enough to get a cell phone! 
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My 10 year old recently asked for a facebook page. When I explained to her that the minimum age requirements was 13 years old she was quite surprised considering some of her friends had accounts. Well, the conversation pretty much moved on from there and we signed up for an email account!