The Fediverse is a network of apps that are compatible with the ActivityPub protocol, like Mastodon, GoToSocial, Misskey, Pleroma/Akkoma, PixelFed, WriteFreely, Funkwhale, PeerTube, Mobilizon, etc. Imagine if you could subscribe to and browse a YouTube channel directly from the Instagram interface, that's the level of interoperability you'll have on Fediverse! But some cross-apps interactions will be degraded (for example, you can comment, but there might be no emoji reactions). There are bridges to interact with content from apps that aren't federated to the Fediverse. There are some community-maintained (example for Bluesky), or provided by company, like Meta that made Threads compatible, but many believe it's actually an evil plan to lure potential Mastodon users, similar to when Google Chat was first compatible with the XMPP protocol and then dropped support after acquiring a user base.
The "Fediverse" (often abbreviated as "fedi") comes from "federation". A common metaphor to explain it is to say "that it's like email": you choose a provider (e.g., gmail.com, belgacom.be, wanadoo.fr), but you should be able to migrate to another provider at any time. Choosing a provider shouldn't limit your ability to send emails to anyone else on earth (but Gmail might decide to block the emails coming from some weird, unknown server)! This is a great feature because, like email, no single company can shut it down or impose unwanted changes (such as a new Instagram algorithm), as long as people are willing to run servers (also called instances), the Fediverse will continue to exist!
Choosing an instance isn't a big deal. Each instance may run a slightly different version of the app interface, some with fancy features (like markdown support or math formula rendering), and they'll have different moderation rules: some are stricter about NSFW content, while others are full of Nazis... Migrating an account from one app instance to another should always be possible without losing posts or followers. Federation is something that your instance's admin fine-tunes. For example, they may (and should) choose not to fetch content or allow interactions from spammy or Nazi instances. However, they can only moderate or ban users on their own instance!
The Fediverse, and in particular Mastodon, isn't the right place for secret conversations. Your DMs are just not visible to other users, but they are not encrypted. If you need real privacy, use Signal. However, be aware that Signal isn't designed for large discussions with hundreds of users (encryption works like clinking glasses with eye contact at a party, if it's a big party, it takes time). This is why group conversations on apps like Telegram are unencrypted by default... The good news? Mastodon (and other Fediverse apps) excel at broadcasting content to a large audience (like your favorite radio station).
Okay, you created your Mastodon account, but there's nothing in your feed, what to do? Mastodon (like most Fediverse apps) has "no algorithm", meaning your feed only shows posts from people you follow, in de-chronological order. There are no ads (aka "sponsored content") and no recommendations ("suggested posts or accounts"). You have to manually search for people and topics, like on the "old" internet. It takes time but it's rewarding because the interactions you build are more meaningful, you'll engage with people who share your passions rather than getting clickbait designed to maximize engagement (and so revenue). If you're a content creator, you'll no longer have to reverse-engineer platform algorithms to "hack" visibility without being shadowbanned, you can focus on actual content quality. Most instances are strongly moderating certain forms of advertising, but allow self-promotion of your art, book, podcast, or even paywalled content like Patreon or OnlyFans!
Twitter (now X) deliberately made it difficult to automatically import your contacts, tools that once did this no longer work. So here are some tricks to get started: ask your friends IRL if they have an account or know someone who does, and motivate them to join. Make it easy for others to find you by adding a link to your account in your other social media profiles or blog. Mastodon has a nice feature that display your website as "verified" if you link your profile on your website (and your website on your profile)! Follow hashtags (on your instance or across instances using https://fedi.buzz), groups of interest (you can find some at https://fedidevs.com/starter-packs, soon something similar should exist as Mastodon's "starter kits" feature), and your favorite blogs, as long as they use WordPress or offer an RSS feed with https://rss-parrot.net! Post an introduction and write a profile description that heavily uses hashtags, so people can easily find and follow you. Look at profiles of other users in your instance (if you think the instance is cool, chances are the people on it are too)!
Mastodon has been the best social network experience of my life so far, but not necessarily because of the Mastodon web interface or mobile app, which I don't find particularly ergonomic. Personally, I love using https://phanpy.social, and I highly recommend giving it a try (and add it as a shortcut on your phone's home screen)! Finally, people on Mastodon generally want you to enjoy your experience because they feel a sense of ownership over the space and want you to feel at home, too. So don't hesitate to reach out! If you ever have doubts, you can always ping your instance admin or moderators' they're usually listed on your server's "About" page. Hope you'll enjoy the ride, welcome to the Fediverse!
Little opinionated introduction wrote by @yvan under license CC BY-NC-SA 4.0 :) If you want to know more, other great resources exist, like: