Because everyone uses it. Once a social app becomes mainstream it gets a giant advantage due to the amount of inertia needed to switch entire social circles to

Because everyone uses it. Once a social app becomes mainstream it gets a giant advantage due to the amount of inertia needed to switch entire social circles to a new platform. Most new social platforms that make it big don't really take over older ones, they just grab a younger generation - usually just by being the network their parents aren't in.