Networks tend to follow an exponential growth rate, so even if the same number moved to the alternative platforms, the time it would take them to grow large enough to be significant would shorten substantially.
Because networks tend to follow an exponential growth rate. The time it takes to go from 1 to 128 equals the same time it takes to go from 128 to 16384, so if you add 127, you've shortened the time for the network to grow from 1 to 16384 by half. Meanwhile, for a platform that already had 16384, subtracting 127 is a drop in the bucket.
> Is there any evidence that this is the case?
Facebook/Youtube/Twitter/et al couldn't have grown as much as they did as fast as they did unless that was the case.