I used livemocha to learn hrvatski. I had a good experience, met a lot of people that were always willing to help. One very positive aspect about this tool is that the learning modules are quite short, you don't have to spend hours to complete each of them, they are well structured. I was able to learn at least the basics, vocabulary, pronouns, adverbs, and so on.
Every time you complete a Writting section, it is shared with "language teachers" who review what you wrote and rate it, providing feedback about your speech.
Even though this site is not available anymore, it has been acquired by hellolingo conserving the methodology.
I was so engaged with the tool that I added myself as expert of my native language to help other people in their learning.
One of the things I really disliked about this tool was that many people were using it to send aggresive comments or sexual content messages. As I am an adult I am able to distinguish them and discard them, but if you think that children can also use the site for learning purposes, it is an aspect to really worry about.
Moderate somehow messages sent by users. Use some kind of software to don't allow sending bad words or sexual content messages.
I was able to quickly learn other language not being alone, you always have someone that knows the language you are learning and is willing to help, so you don't feel alone during the learning process.