And you forgot a big one - Pinterest, which is larger than Snapchat, thriving globally and worth $26 billion. Facebook has been entirely unable to slow their growth.
I think it's clear at this point that Facebook will continue to lose their social dominance, so long as they're prevented from acquiring any major competitors. Their old-line social network will likely remain as a foundation that everyone joins, because so many people are on it. New networks will keep popping up for the same reason TikTok did, young people hate being where old people are, where their parents and teachers are. That cycle will perpetually repeat (in 15 years TikTok 'dancing' videos will be viewed as laughably cringy by young people, like 1980s big hair and fashion trends were 15 years later). All that's necessary is to prevent Facebook from buying the new networks as they pop up.
Instagram might even manage to kill itself trying to be too many things as it clones competitors, turning itself into the F35 of social networks.