If it’s true that italki has 5 million users and 10,000 teachers, then only 0.2 % of the users are teachers. That’s hardly „all members“.

To become a community tutor isn't that difficult. But, as Gabor said, then you have to find students, and if your teaching and/or language skills are not that strong (being a native speaker does not necessarily mean that you can teach people that language), then finding and retaining students will be a struggle.

Becoming a teacher is not hard at all, but after that you have to find students, which is much more difficult. :)

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There’s a shadow price for every language exchange (trade) you only need to do the math.

This guys probably already have the matrix at least for the most common languages.