Mastodon is not the name of the network, but one of the implementations of ActivityPub.

Moreover, ActivityPub services are not limited to microblogging. There is also an alternative to YouTube → PeerTube (e.g. https://v.lor.sh for users of our instance), Instagram → PixelFed, and Reddit → Lemmy.

Can you imagine being able to read, write, and repost comments on a YouTube video without leaving Twitter?

It's possible here. For me, that's the real killer feature.