Mastodon is a federated system like StatusNet/GNU Social.

So, in your opinion, Mastodon nodes - by virtue of being federated - would be better equipped to handle the spam and harassment volume that Twitter is subject to?

I find that hard to believe.

ActivityPub (and OStatus, and ActivityStreams/Salmon, and OpenSocial) are all great specs and great ideas. Hosting and moderation cost real money (which spammers/scammers are wasting).

Know what's also great? Learning. For learning, we have the xAPI/TinCan spec and also schema.org/Action.