Instagram doesn't have the same network effect as Facebook. Snapchat does have Quick Add, which shows people you might know; it's also a much more communication-driven platform.
TikTok is probably the modern Instagram replacement, not the modern Facebook replacement.
The barrier to making TikToks is much higher than for Instagram posts. It takes hours to make a decent TikTok; it takes second to make a decent Instagram Story post. For that reason, I see TikTok as more adjacent to YouTube than Instagram.
TikTok is probably the modern Instagram replacement, not the modern Facebook replacement.