Just recently thought I'd try out Bluesky due to whatever recent Twitter drama is going on (I don't seriously use any microblogging platform beyond messing around) and I found it quite nice, so I've recorded my initial thoughts here for SEO (and also to test the new blog updates).
Additionally, I've stuck a link to my Bluesky profile in the header for the time being.
The OG platform-turned-garbage-heap. It was OK back in the day and its main value was introducing people to the concept of "microblogging", made more fun by the fact that, at the time, you only had 140 characters to work with per message, encouraging clever word usage.
Nowadays, the platform has been enshittified - you must have an account to see anything other than one message at a time, and it aggressively suggests you should register on every page you're on. You can pay for a subscription that does... something. Might give you a little kickback for reach/interaction? Unsure, but it's a microblogging website. Why is that even necessary?
Your "handle" (or, the "username" as some people call it, the part after the @) is globally unique, and is how users refer to you. It's formatted like @usernamehere. The opportunity for vanity handles is slim, but there are some nice examples out there.
The UI is historically decent, but definitely funnels you to content that was paid for. At least the ads aren't too intrusive.
Fediverse
The runner-up in terms of longevity, the Fediverse is comprised of most software that supports ActivityPub - so, Mastodon, Pleroma, Sharkey, Misskey, FireFish, and even some reddit-likes like Lemmy, Kbin, Mbin, and so on.
Almost all of these software can interact with no major loss in featureset, meaning that your theoretical audience on any one instance could be the rest of the fediverse.
In practice, you're mostly limited to the instance that you've registered on unless you (or the instance admins) go out of your way to follow people outside your instance or force federation with other instances. Those instances are generally pretty silo'd into a specific ideology or set of topics, because if you don't like what an instance is about, you would register at a different one. As a result, they pretty often feel like echo chambers of more-or-less the same regurgitated content made with different words, because that's what the people on that instance like. Bullying people off of instances, or (in the case of the reddit-like) outright blocking people just because of what instance they're registered on is more common than many folks would like to admit.
Your handle is globally unique, and is typically formatted like usernamehere@instance. This gives users a good opportunity
Overall the fediverse options offer the best theoretical reality, but is way stunted in both reach and diversity-per-instance.
Bluesky
Preface: I have barely used Bluesky, there may be more nuance to it, but this is what I've observed thus far.
Bluesky is nice. The UI is snappy and responsive, works decently on mobile.
Functionally very similar to Twitter, in that you have a central information hub. The userbase is smaller, so the amount of information is also smaller, but there seems to be a pretty decent variety still. You can run a Bluesky server on your own and federate with the main hub, but it doesn't actually look like there's much benefit to it.
Bluesky does not federate over ActivityPub (at this time).
Your handle is globally unique and is typically formatted like @usernamehere.bsky.social, but if you own your own domain and can set some DNS entries, you can have your handle be any domain name. For example, mine is @andrewph.org. It's a little more confusing, IMO - matching ActivityPub's actor stuff would have probably been a bit easier to adjust to. Everybody's used email, after all.
Conclusion
If I had to rank the 3 platforms, first would be Bluesky, second would be Fediverse, and third would be Twitter. My ranking is based on a combination of usability and content, with the Federated options being ranked higher than Twitter because of the massive amount of functionality and flexibility it offers.
Twitter loses points for swirling the toilet drain.
Everybody knows that pi.andrewph.org is the best microblogging platform anyway.