I played around with it and it’s basically useless without a follower only feed. And the posts tend to just basically disappear forever after a feed refresh.
But if they follow through on ActivityPub integration I’ll be stoked to follow all the normies that couldn’t get by on mastodon that are using threads. More content = more better.
I won't be happy if they integrate federation. Ever heard the phrase "embrace, extend, extinguish"? It's a tactic used by large companies to squash growing competition.
Google used it, for example, to squash a growing open-source chat messenger protocol called XMPP. (Think of XMPP like ActivityPub.) Google allowed its Google Talk application to integrate with people using XMPP. (They embraced XMPP.)
Then, they added their own proprietary features that wouldn't work with normal XMPP users. (They extended, or built on top of, XMPP.)
Then, they cut support for XMPP integration, leaving it effectively dead in the water. XMPP users suddenly had a list of Google Talk users in their friend list who would never appear online again, whereas Google Talk users maybe had one or two people in their friend list who looked like they'd moved on from Google Talk. (They extinguished XMPP.)
Now imagine that happening with Threads. You, a Mastodon user, follow a bunch of people who just happen to be on Threads. There are some things Threads users can do that you can't, but you don't really mind. It works well enough. Then, one day, Threads stops working with Mastodon. Suddenly, over half of the people you followed are no longer available to you. The only way you can follow them again... is to join Threads.
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.