Each time an exodus happens in reality the users 'said' they would leave only come back after a few months. Mastodon did not gain significant retaining users after #deleteFacebook, #deleteInstagram and #leavingTwitter trended.

It is not "early days" and anything compared to being 'A viable Twitter alternative' other than Facebook and Instagram has only shown themselves to be a complete failure. Just like how Gab (another alternative) with roughly the same number of active and registered users as Mastodon also failed to be a viable Twitter alternative since that has instead turned into another echo chamber but including the undesirables putting off many users on Twitter from moving.

Mastodon as far positioning itself as being 'A viable Twitter alternative' after 6 years of existence has failed to gain any significant amount of former Twitter users or traction. After 6 years, it isn't early days.