Currently, with all social networks and internet, it seems that everybody suggest  that the primary way to learn a language is to find a language exchange partner.
Beyond the obvious disballance problem (some languages have more learners than others, so finding a partner for everyone might be impossible), I think that it also decreases the flexibility. If I learn alone, I can do it whenever I have time.
Is there an evidence that such approach is significantly more successful than variants like:
- after getting basic from school/classroom training, learning alone using internet materials, Anki/SRS, graded readers,
- classroom learning, even if the teacher is not a native speaker (or even better, if is), and I understand that this has significant financial cost
- language speaking clubs, where several learners meet weekly/monthly and try their best speaking L2, even if no native speaker is present
Language exchange partner will be unlikely a qualified teacher, and for many languages (especially English) there is almost too many learning resources and some are of quite high quality (so many that it might be hard to find the really good ones)
I am not against speaking, I am concerned that placing all bets on a single approach increases risk of failing ("I could not learn because I found no partner").
So is there evidence about how successful is "language exchange" approach as compared to old-school alternatives?