Misskey is most similar to Mastodon.
Bluesky is also a bit similar to both but still the most different one from the other two. All three are different social networks. Mastodon and Misskey are ActivityPub compatible, meaning they can “speak with” other ActivityPub compatible social networks, e.g. Lemmy, Pixelfed or PeerTube. Together, that’s what’s called the Fediverse (different federated social networks being able to talk to each other).
Bluesky is based on a similar, but different protocol called AT Protocol. It also means it’s possible for social network services using this protocol to be compatible to each other. But not sure if it’s there in practice yet, if there are even other social networks using this, and so on.
I’d recommend using Mastodon, and in general ActivityPub compatible social network services. They’re all open source, anyone can host a server (which is very important, because if the server operator ever does any bullsh!t you’re not forced to stay there and still can remain on the social network, just from a different node), they’re federated (servers can talk to each other and usually do unless some specific servers are blocked on purpose by the other server), and they’re compatible with multiple services also using the same protocol. And there are “big” networks already existing using ActivityPub, most notably Mastodon and Lemmy of course. Also, Meta’s Threads is also using ActivityPub, however some Mastodon instance hosts have decided to block Meta’s servers (there are good reasons for doing so but explaining this would make this post even longer).
Bluesky is controlled by a company made by ex-Twitter employees, I think its federation capabilities are still limited right now(?), and one can’t be so sure how its future is going to look like under these circumstances. We’ve all seen what happened to Twitter after Musk bought it, so I think only the true, unrestricted open source social network platforms like Mastodon, Lemmy, Pixelfed, Loops.video, PeerTube and so on are the future.
https://itsfoss.com/bluesky-vs-mastodon/ This is a comparison about Mastodon vs Bluesky.
Misskey is most similar to Mastodon. Bluesky is also a bit similar to both but still the most different one from the other two. All three are different social networks. Mastodon and Misskey are ActivityPub compatible, meaning they can “speak with” other ActivityPub compatible social networks, e.g. Lemmy, Pixelfed or PeerTube. Together, that’s what’s called the Fediverse (different federated social networks being able to talk to each other). Bluesky is based on a similar, but different protocol called AT Protocol. It also means it’s possible for social network services using this protocol to be compatible to each other. But not sure if it’s there in practice yet, if there are even other social networks using this, and so on.
I’d recommend using Mastodon, and in general ActivityPub compatible social network services. They’re all open source, anyone can host a server (which is very important, because if the server operator ever does any bullsh!t you’re not forced to stay there and still can remain on the social network, just from a different node), they’re federated (servers can talk to each other and usually do unless some specific servers are blocked on purpose by the other server), and they’re compatible with multiple services also using the same protocol. And there are “big” networks already existing using ActivityPub, most notably Mastodon and Lemmy of course. Also, Meta’s Threads is also using ActivityPub, however some Mastodon instance hosts have decided to block Meta’s servers (there are good reasons for doing so but explaining this would make this post even longer).
Bluesky is controlled by a company made by ex-Twitter employees, I think its federation capabilities are still limited right now(?), and one can’t be so sure how its future is going to look like under these circumstances. We’ve all seen what happened to Twitter after Musk bought it, so I think only the true, unrestricted open source social network platforms like Mastodon, Lemmy, Pixelfed, Loops.video, PeerTube and so on are the future.