Okay, it's time to tell new users a secret about Mastodon.
Mastodon is (whispers) part of a much bigger network 😮
Mastodon servers use an open standard called ActivityPub to talk to each other. That's how you interact with people on other Mastodon servers.
But, dozens of other federated networks also use ActivityPub, so (this is the good bit!) you can follow people on these other networks from your Mastodon account!
Together, Mastodon and these other services form... The Fediverse
(whispers) Click on the link...
The link you just clicked on leads to a video on PeerTube, on a server called framatube.org.
BUT... if you copy and paste the same address of the video into the search box in Mastodon, and search for that link within Mastodon, it will appear to you as a Mastodon post
You'll then be able to watch it (of course), but also you'll be able to comment on the video by replying to the post within Mastodon, like the video by favouriting it, and follow its creator by following the account that posted it.
This is the Fediverse in action, totally different services interacting through a common open standard.
It can be hard to get your head round this at first, but it's incredibly useful and once you get used to it simply how things should be.
It depends what you mean by "use".
You can follow and interact with these services from your Mastodon.
If you post a photo on Mastodon, it will be visible to people on PixelFed as if you had a PixelFed account.
If a PixelFed person posts a photo, it will be visible to people on Mastodon as if they had a Mastodon account.
There's not a huge difference there in functionality, so there's not really a need to have separate accounts.
In the case of BookWyrm, if you want to write a new book review yourself you would probably need to sign up on a BookWyrm server.
But if you just want to follow and comment on what BookWyrmers are posting, you can do that from Mastodon.
There's no central Fediverse account system because when you centralise stuff, it sooner or later gets bought out by Elon Musk type people.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!