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Meta Threads engagement has dropped 50% in a week
(www.nbcnews.com)
This is a most excellent place for technology news and articles.
ActivityPub is open source. This means, by definition, Meta are allowed to use it. They could easily do the extinguish part by just never using the protocol. If another platform existing is such a threat to the Fediverse then we're doomed to failure in the first place.
While Google removing XMPP no doubt helped sink nails into the coffin, WhatsApp played a bigger role in the death of XMPP than Google removing it from Talk. It was increasingly irrelevant for a now growing number of people.
The point of "embrace, extend, extinguish" is primarily to get rid of any potential future competition. So, for example, it would not work if Facebook tried to do it with Twitter since the latter already reached critical mass and can stand on its own without outside help, barring bad leadership. It typically only works if the party that's being "embraced" is smaller than the company that's "embracing".
To your point of ActivityPub being open source: that's true, so Meta is, legally speaking, free to use it. However, barring any restrictions in the license, they're also free to add any proprietary features to Threads that don't work on Mastodon. It's similar to Google Chrome: Google has their open-source Chromium web browser, but they also have a version called Chrome that has features that are not open-source.