I'd look at it a different way: "Facebook" and "YouTube" aren't single products, but rather each one is a collection of something like 10-20 products that live on a single platform.
For "viral videos", Facebook and YouTube (and Reddit) absolutely directly compete in the same market.
For "educational videos", YouTube competes with other platforms.
While for "tracking social acquaintances" Facebook competes with the Contacts apps by Apple and Google, as well as other messaging apps.
Really there's no such product category as a "Facebook" or a "YouTube". And Facebook's original product category -- to keep track of social contacts and post public and private social messages -- is now only a tiny part of Facebook.
For "viral videos", Facebook and YouTube (and Reddit) absolutely directly compete in the same market.
For "educational videos", YouTube competes with other platforms.
While for "tracking social acquaintances" Facebook competes with the Contacts apps by Apple and Google, as well as other messaging apps.
Really there's no such product category as a "Facebook" or a "YouTube". And Facebook's original product category -- to keep track of social contacts and post public and private social messages -- is now only a tiny part of Facebook.