Hello dillemme et al,
Lang-8 is a free service, but donations are accepted. Also it’s possible to buy premium membership (and practice writing in a huge number of languages) for less than four dollars a month. In addition to all the languages offered here, there are other languages like Catalan, Hebrew, Greek, Arabic, Georgian, Basque, Irish, and Esperanto. I counted about 80 languages. I didn’t see any Native American languages (sad face here), but there are some languages from the Indian subcontinent, Swahili, Yiddish, and Thai for Hape…
Certainly it would be easy to use both LingQ and Lang-8 as a way to improve writing skills. It just depends on what the specific learner wants or needs.
Here’s the rub/downside, though. Some languages have plenty of tutors (English/Japanese/Chinese), while some common languages like Spanish and French are less represented. In any case, I have almost always received corrections and/ or comments within twenty-four hours all for free and all with the kind of correction that I like. The native speaker will rewrite unwieldly sentences, so I have a better idea of how to express myself as a native would (not just as a learner of a second language).
The nice thing is that I can choose the writing that I would like to correct. I don’t have to correct writing about a topic that I find offensive or a topic that doesn’t interest me. Like you, dillemme, I have corrected more than I have written, but that’s my problem. I like to correct. I probably correct 4 or 5 compositions for every one that I write!
There is also a nice option. Members can give memberships to other members who have helped them.
In short, for people who want regular interaction with French, German, Spanish, or Russian speakers, I would recommend LingQ first, then Lang-8. For people who want to practice writing Japanese, I would recommend Lang-8 because there are a lot of Japanese members and the writing will be corrected quickly. If you don’t mind having other people see your writing and mistakes, Lang-8 is a good option. If you would prefer that your writing not be seen by the entire internet if there is a bug, then LingQ is better option.
I hope this clarifies some of the differences. I also belong to Livemocha.