I guess this is subjective, but I don't really consider Mastodon to be (very) decentralised; it's federated, like email. Your account exists on only a single node—with that account you are free to communicate with any node, but you still have a single host/entity maintaining your identity.
If you want an example of a working decentralised social platform, have a look at https://www.scuttlebutt.nz/
> However, I have not found that it works particularly well as a decentralized social platform. 100% of the people I follow and the people who follow me are on the mastodon instance I'm on
While I like the idea of proper decentralisation, I don't really see the problem with what you describe. Mastodon seems like a success even if every user only follows other users on their own instance. The point is that it is potentially possible for you to migrate to another provider and still follow the same friend list—vendor lock-in isn't possible. Just like email.
This isn't perfect of course—we almost had this with Facebook and GoogleChat but they both gained such a critical mass of users that they could shutoff XMPP support without user backlash, and Gmail also maintains a similarly locked-in status even while supporting federated standards (IMAP/SMTP) simply by making that support, and migration, needlessly complex.
This is why decentralisation matters. Federation is still pretty cool though.