Some users are above-average draws for others, and if a critical mass of these important nodes in the network map depart for a new service - like, say, #Mastodon - that service becomes the presumptive successor to the existing giants.
When that happens - when Mastodon becomes "the place we'll all go when Twitter finally becomes unbearable" - the downsides of network effects kick in and the double-edged sword begins to carve away at a service's user-base.
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