Mastodon has around 1 million active users³ Bluesky has around 3.5 million active users²

Bluesky doesn’t have a decent way to see active user count, but it is likely higher than 3 million

Mastodon retains 10%, Bluesky retains 10% also, but I can’t confirm it

Edit: Using unique likes, it shows about 2 million active users on each day¹

Source:

Bsky Analytics¹ • Bsky Stats² • Mastodon Analytics³