Lang-8's particular take on this is with an emphasis on blogging. These entries may be as personal or impersonal as you feel comfortable with. There is a lot of important vocabulary rolled up into writing about that bird incident while you were walking your dog this morning, or how wonderful this new corner café is. In addition, the sort of voice you'll be learning to use is a natural, idiomatic, casual tone that many formal language courses fail to teach entirely, focusing on the slangier, more conversational side of a language that real people really use.
What happens with all these blog entries is that other users who are native speakers will go through and reply to the blog entries, including not only friendly comments, but also valuable edits and tips for improving your use of the language. In return, you are expected to help other people improve their use of your own native tongue by commenting on their entries. It's a simple exchange, yet a powerful one.
(If you aren't into blogging but like the idea of social learning, there are many other social learning sites available that emphasize different social interaction, for instance, Livemocha and its emphasis on learning through IM conversation.)