[kidlogic]

Platforms require two important factors - self-marketing and publishing tools. It's important that the published content benefits everyone within the community, which helps build up to true Network Effects.

Github (same-sided platform): publish your repo, users can download and contribute to your code. More code, more branches, repeat.

Youtube (two-sided platform): Published content provides entertainment. Users can respond to others video to provide additional relevant content. Each publisher promotes their own channel for recognition (and in turn, promotes your platform).

I'd recommend reading: http://platformed.info/ The author really dives into Platforms vs Marketplaces and how to increase your chances for success. It's the only blog I've ever subscribed to.

[kromodor]

This seems pretty sound.

It should provide benefit of publishing inside regardless of the inside crowd (to a point of course).

Another example like yours - soundcloud.