The Premise (4 out of 5)
Social learning is, essentially, learning a language through social interaction, through everyday communication with native speakers in whatever language you'd like to learn. It provides practical foreign language skills for travel and living abroad, and is more motivating than the typical repetitive grammar exercises. It's an admirable premise, and while it may not be teaching you the French of Camus or the Russian of Dostoevsky, you will learn everyday language skills in a fun and fascinating manner.
Using Lang-8 (5 out of 5)
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.)
Getting Started (4 out of 5)
Of course, it's impossible to jump right into writing even semi-fluently in a language—it takes most students at least a year to get to that point for languages similar to their native tongue. We're not even talking about the slangy, casual voice blogs most people prefer in blogging, or in languages structured completely different from their own. While other users jump to help correct your entries, Lang-8 offers little by way of help here for absolute beginners to get to the point where they can even begin writing. It is advised to be at least an intermediate speaker of the language, with the ability to conjugate basic verbs and so forth, to really get anything out of the Lang-8 experience.
If you have enough knowledge of a language to immediately start blogging, then you'll find it very easy to begin. Even if you don't already know people on the site, many people are very eager to make new friends with complete strangers. People are very inquisitive about other countries, and I have yet to hear any complaints of stalking as with many other social networking sites.
Usability (4 out of 5)
While getting started might not necessarily be that easy, once you're in, the site is very easy and intuitive to use. The layout and interface is similar to that of most other blog-based sites and is simple. You can sort entries by tags and by language, allowing you to quickly find blogs that might interest you in whatever language you want to learn (or practice). It doesn't boast of too many additional features—so don't be expecting anything like Facebook's plethora of apps. However, it is fun and functional at what it does, and that'll be plenty for most users.
Effectiveness (4 out of 5)
As with any language learning program, fluency won't happen over night. Lang-8 requires patience and a certain lack of pride: You have to be willing to make mistakes and to accept corrections from native speakers, and you'll be at it for a long time before there's noticeable improvement. However, as long as you stick with it—which is easier to do than with other programs, if nothing else because it's simply more fun—you'll find that fluency creeping up on you, entry by entry.
Variety Of Languages (4 out of 5)
While you are not restricted from writing in any language at lang-8, it is obviously easier to receive comments in that language if there are more native speakers on the site. Learning more obscure languages, thus, might be somewhat difficult through this method. Common languages like Japanese, French, Spanish and others have a strong presence on the site, which in turn leads to a healthy pool of possible commentators for your own blog entries.
All in all? I'd highly recommend using Lang-8 as a fun supplement for learning any language, once you've hit an intermediate or moderately advanced level of fluency. However, absolute beginners will likely find themselves more frustrated than helped in attempting to plunge into full scale discourse with so little knowledge of the language.