It’s using ActivityPub. It’s interesting because Bluesky (basically, former Twitter) explicitly stated that they didn’t think AP was robust enough and created their own shitty protocol. Facebook might be one of the first traditional SNS platforms to experiment with AP. I would’ve been happier if it were Tumblr, who said they were working on implementing AP last year.
I wonder how much of the recent Twitter drama has influenced Facebook’s decision to release Threads so soon after its announcement. It’s probably going to be shit quality and just try and monopolize on people leaving Twitter but not wanting to jump to Mastodon.