While LiveMocha contains predefined lessons, italki allows people to use an instant messenger app to talk with others via text, voice, and video. Both sites are excellent; livemocha provides self-study lessons that are important for learning to speak a new language and italki allows you to talk with native speakers. To effectively learn a new language, I recommend using both sites.
Italki allows you to create groups, create a wiki or language pages, for example, you can post lesson information on a page and students can submit their homework via a comment box. The main difference between the wiki and the page is that anyone can edit a wiki whereas you control who edits your page or pages. Both are useful. The wiki option is great for coursebooks where everyone can share their knowledge.
Italki also allows you to upload files. I uploaded my ESL PowerPoint lessons that I use in class.
Within 15 minutes of joining italki, I made 2 friends. One initiated a chat session where I answered his English questions and I learned some basic Portuguese.