#Bluesky is way bigger, but I can only see bridged accounts here. I think, it's the second biggest already.
https://bsky.app/profile/fingolas.bsky.social/post/3lco3g5ojak2m
Indeed - if somebody follows you from Bluesky but their profile is not bridged, your Mastodon instance has no way of knowing that they follow you as it is never communicated across protocols.
So for Bluesky specifically you might want to count manually to get an indication of your reach. Any other service should be fine as it is.
@mho
Edit: It is however worth noticing that a lot of followers on Bluesky appear to be spam bots or similar trash accounts. It's hard to measure genuine engagement.
@mho you made bluesky blue