PikiFriends is a wonderful site for introducing web 2.0 skills via an online "pen-pal" system. This is ideal for education because it is being filtered and teachers have a lot of control over student accounts. Also, there are a number of great resources on this site and an already well designed curriculum that helps integrate PikiFriends into a classroom.
I highly recommend checking out PikiFriends by clicking here!!
BTW, this is designed for Jr High & High School students.