Platforms such as Facebook are great aggregators of generic information, but are not very customisable when you want to discuss specific topics i.e. when people want to talk about cars, Twitter or Instagram are limited to comments when in fact you might want to have more features to generate interaction. They also don’t want to discuss it in the same place as their uncle.
I believe this is the point we are now: a lot of people have now discovered the positives of discussing with people alike and are searching for places where they have more of their kind. This is why Reddit, Twitch or Discord are growing strongly, unlike Facebook.
While more and more people are still joining the platforms, the future will bring other smaller places for niches. Social Media is not saturated, it’s fragmented.