I've been using Mastodon for some years now and I can't praise it enough. I believe mainstream social media has become a cancer on our society, but not because being social is inherently bad, but because people have allowed themselves to be herded into social spaces controlled by for-profit companies that are farming them for their personal information and then force-feeding them targeted marketing materials for profit with algorithms that are opaque and out of the user's control. Most people I talk to say they stay with their social media platform of choice because all of their friends are using it. I decided long ago that wasn't enough of a reason for me and I'm here to say that you can still enjoy an online social experience outside of that paradigm.
Mastodon is different. When you first sign up you see NOTHING. You have to discover and create your social network, it takes some effort. Most of the people you know in real life are probably not using it (though many are starting to use threads and bluesky which promise to be reachable someday - don't hold your breath). But once you do start to create that network, the information you see there is provided by the people you follow, it is not auto generated by an algorithm, and there are no ads (except if you follow people who post ads obviously but thats your choice).
Mastodon is not centralized so it does take a bit of a mind shift to understand how it all works. People who are used to traditional social media expect that there will be a single website that you sign up on, and many new Mastodon users do sign up on the main mastodon.social site itself, but anyone can run the Mastodon software, and very many do so there are many Mastodon "instances" and they all have different domain names, and many (but not all) of them are linked (federated) so that you can follow and see posts from someone on another Mastodon server. You can also set up a new Mastodon account on more than one server and you can always move to another one taking most of your information with you so you don't have to worry too much about picking the wrong one. mastodonservers.net has a database of known Mastodon servers you can review.
The owner of a Mastodon server can control many aspects of how the server works and which other servers it connects to. If you really want to end up in an echo chamber that is certainly possible if you pick a server that is very isolated. Some mastodon owners might welcome LGBT folk while others might ban users who post that content, some owners might offer membership only to people with certain interests or credentials, others are open to all. You could, for instance, join a mastodon server run by someone with a particular interest you share. The members of that server might regularly post stories about things you enjoy so joining that server might be a fast way to build an enjoyable social network because discovering people with your interest is easier since there are many folks with that interest already in one place.
I have several mastodon accounts on different servers and I follow different folks on each. My identity on each server is still me, but presents the public persona of me that is interested in that specific hobby, pastime, interest, etc. You don't have to do this. You can certainly follow lots of folks with different interests from different servers, but its an option (just like Mr Whiskers your cat has an account on facebook).
Wil Wheaton famously had a bad initial experience on Mastodon and I think its important to go read that but I think that was more related to Wil's notoriety and fame than how Mastodon itself works. If you are finally leaving X (I left a long time ago) here's a good article about why bluesky is not a good alternative to X. If you do set up a Mastodon account, feel free to follow my Fosstodon account linked in the socials below. Fosstodon is an invite only Mastodon instance for those interested in technology and open source software in particular.
update Dec 1, 2024 - Freaking excellent (and super long) blog post by one of the architects of the ActivityPub spec (the Mastodon/Fedieverse "way") talking about how it differs from ATProtocol (Bluesky "way") and whats good and bad about both here: https://dustycloud.org/blog/how-decentralized-is-bluesky