Lang-8
Target Audience: High School and Up
Link: lang-8.com/
"Lang-8 is a free language-exchange social network. Just write a journal in any language you’re learning, and it will appear to native speakers of that language. They’ll correct it and leave useful comments and feedback. In return, you check other users’ journals written in your native language. By checking each other’s journals, you can improve your skill in any number of languages, help others to master your native language, and make a lot of friends along the way.
Language learning doesn’t have to be done with dusty textbooks anymore! Let’s make some friends and learn some languages on Lang-8!" (source)