In my opinion there is two way to efficiently learn Chinese online both on the Writing and Oral side
[*]Writing with Skritter.com
Skritter is a tool for learning Chinese and Japanese writing with stroke-level feedback, tone practice, audio playback, definition practice, and progress feedback. It's extremely fun and very efficient, you can possible expect to learn 1 to 2000 characters within a year.
[*]Oral with Chineseteachers.com
To simplify, Chineseteachers.com is like a place where students, meet real professional native Chinese teachers located all around the world (and therefore mastering different other languages such as English (mandatory), French, German, Japanese, Portuguese…etc.).
Once you’ve registered, filled your profile (better to let your teachers know who your are) and topped-up (you can start with an amount as low as 1$), you simply go to the main pages and check which teachers are already online:
As said earlier, there is no booking needed and I can start a lesson right away without having to install any software, everything is made through your browser (make sure to use the latest one such as Firefox or Chrome).
The good point here is that it's a private one to one lesson, therefore during a 2h lesson, I really speak for two hours.
Have you even tried these services? Do you like them?