There is technically still an option on the official app to "pick my own server", but it's much harder to read or notice, and new users will probably click the mastodon.social button without even understanding the other button.

Mastodon.social just passed 1 million registered users about a week ago, out of a total network of approx 7 to 11 million users (dependiing on how you count them).

Most of these users probably signed up via the official app.

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