Have you checked out these useful resources yet?
Think U Know is a great website for children and young people – there are pages that cover children aged 5-8, aged 8-10, aged 11-13 and aged 14+. There are also really useful pages for parents/carers.
The NSPCC also has great guidance to the social networks your child might be using.
Finally (for now!), Vodafone has been particularly supportive of parents with their Digital Parenting magazine. They’ve produced this for five years now. The magazine is available as a downloadable pdf.