[Benjammer]

I believe this use of the word "platform" in this context is a much more specific meaning where it's a counterpoint to the idea of an "aggregator." It comes up in discussions about modern large tech companies being "aggregators" or "platforms," and this is the point (allegedly) Gates is making here, that Facebook is much more an aggregator of content that users bring into the walled garden than they are an economic platform for other businesses, which is what Microsoft was/is.

[TeMPOraL]

If you believe that all this advertising spending and social media marketing activity actually adds more to companies on Facebook than it does to Facebook itself, then you sort of could call Facebook a platform per this definition. An advertising platform.