[thaumaturgy]

I think most people crave community and especially for something like kicking a tenacious habit, some social support could be really helpful. Of those options, probably a very simple forum would be best.

Chat is too dependent on having a critical mass of people online at the same time. (For a niche site I visit occasionally, online-go.com, there are several hundreds of thousands of users but it's not unusual to see only 30 or so in the chat at any given moment, and of those, only two or three will be active.)

Q&A has to have some kind of purpose behind it. SO became a hub for programmers and people in related fields who had to solve specific problems on a regular basis, often the same problems over and over. It kind of suffers as a community support thing and shines as a repository of knowledge.

"Rooms" are just a partitioning of your chat or forum, and probably shouldn't happen before they become necessary.

But be prepared to spend a significant amount of effort on whatever social layer you build. Whenever a bunch of people start to interact, there will be differences of opinion and then disagreements, malicious people, well-meaning but disruptive people -- handling all that can be a huge energy sink.

[GeoffreyKr]

Thanks for this complete answer, I must say that I agree with your points. Have you some example of well-done forum on mobile apps ?

[thaumaturgy]

I don't, sorry.

Maybe you'll build a good one. :-)