This market resolves to YES if:
Officially reported (or best available estimate) of Monthly Active Users for any part of 2030 are higher for Mastodon than Twitter.
Twitter doesn't exist in 2030 but Mastodon does and has at least 2 million active monthly users* at any point in 2030.
And NO in every other case.
*The number reported by them in November 2022. If they don't reach at least that in 2030 and Twitter doesn't exist then effectively they have both failed as far as this question is concerned.
I wonder how it would work for Fediverse vs Twitter instead of Mastodon vs Twitter. Mastodon is the most popular fediverse network by far, but the compatibility of fediverse clients/servers makes it easy for users to migrate to something else, while staying a part of the overall network.