According to the most recent Streamlabs report: YouTube Gaming Live accounted for 22.1% market share for 2019 in terms of hours watched while Twitch market share accounts for 75.1% and Mixer market share accounted for is 2.7%.

Youtube Gaming is also growing extremely fast since they have rolled out their new creative suite.

https://newzoo.com/insights/articles/streamlabs-newzoo-q4-ye....

Facebook Gaming hardly has any market share so I don't understand why you are saying their existence and networking abilities is enough to make alternativeS unviable. The streaming industry seems to be one of the most competitive markets out there.