In my personal opinion, italki is primarily a marketplace that matches students with teachers who give paid lessons over the Internet. That part of italki is good, and my experiences with teachers have been good. You can learn a language that way. But it is completely pay-for-service, and teachers charge what they charge. 

Community tutors charge less, but they also charge.

italki also provides a mechanism whereby people can meet language companions for language exchanges. This part of the service has been valuable and interesting to me, and it costs nothing. But there are no guarantees, many people are not sincerely interested in language exchanges--and you can only find a partner if someone is interesting in learning your language, and some languages are more popular than others.

The "discussion," "answers," and "notebook" sections are only fair. They are there. They are what they are. They are not in themselves a way to learn a language.

In short, italki is a good place to buy paid language lessons from individual teachers. It is not a tool that will teach you a language for free.