if they've got furries in the top six there must be a thriving CP community on there

If you count lolicon as CP, then yes, it's run by Pixiv.

Mastodon looks like twitter, but it is not twitter. It a is decentralized system where anyone can run a Mastodon server, and specify which other mastodon servers that server communicates with. The result is that like-minded groups will see eachother, and only eachother, and basically nobody will ever interact with anyone their server admin doesn't agree with.

Needless to say, you can tell it was designed by leftists.