Threads is implementing it in phased rollouts and I think they saw the writing on the wall with X that Bluesky was the next "big thing" and wanted to jump on a competitor protocol that had already been developed and already had an active base of both users and developers.l, whereas Bluesky is building everything in house from the ground up with the AT protocol.

WordPress has a plugin that is developed by Automattic (as close to core WP as you can without actually being core WP) which essentially turns every WordPress site into an ActivityPub feed. Its really cool and an incredibly powerful tool for publishers.

Flipboard is also implementing ActivityPub as we speak, and it seems like they are quite bought in on the concept. Their CEO hosts a podcast about the Fediverse.

Ghost is a publishing platform similar to Substack that is also working to implement ActivityPub and is doing a lot of the heavy work in terms of trying to figure out what longer-form publishing could look like within the fediverse, as opposed to being a network of different Twitter clones.