What Makes A Service A Platform?

I have been told - or read - that a platform is totally ignorant of the content on the platform. Under this definition a word processing, spreadsheets or YouTube is a platform. Right?

What about the fact that these platforms define the type of content? Word processing is usually mostly text - words. Excel is almost always numbers and formulas. YouTube is videos and only videos.

Does this keep these from being platforms?

Is Facebook more of a platform because you can have text, pictures, videos, links, messaging, etc all on one service?

Twitter is text and links only. But I think most folks consider Twitter a platform because it has standard API's for third party developers to use. Is this a part of the required criteria for a platform?

Any opinions?