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…
It should provide benefit of publishing inside regardless of the inside crowd (to a point of course).
Another example like yours - soundcloud.