Yes, it's very possible to have such platforms fully decentralized. However, I believe you won't expect that to happen because non of them will interested in going towards that direction. This is because full decentralization often leads to loss of control and non of those billionaire owners would want to lose control of the platforms they have built over the years that have made them what they are today. Yes, YouTube and Facebook can be fully decentralized but that's not going to happen.