I'd be the last person to downplay the fact that the Bluesky app has a serious retention problem. But it has "broken through" in an incredible way and DAUs/MAUs are quite stable.

Registered users is not at all meaningless. Bluesky has those user's email addresses, the mobile app is still installed on many of their devices, they have accounts, and they can potentially be reactivated.

For example, if Bluesky announced a feature exciting enough, like subcommunities, it could email those 50M users and possibly bootstrap a serious open network competitor to Reddit.