The tricky bit is that it's logically decentralised in two ways. In that there is no central Mastodon instance, and there is no central ActivityPub implementation either.
Maybe there doesn't need to be a quick way to compare it to something else?
In my opinion, Mastodon isn't the end goal for social networking. It may just be the stepping stone for what could be the "social web".
What is more important is that this social web is based upon a standard like ActivityPub, where it allows different platforms (like Mastodon) to interact with each other.
A way to imagine it would be if Twitter, Facebook, Tumblr, LinkedIn, etc. were all based on the same technology. You join each platform, but at a fundamental level, they work interchangeably.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!