I don't trust Meta/Facebook even a tiny bit. We on the fediverse shouldn't implicitly support Threads by interacting with it at all. It gives it legitimacy. It encourages users to get their data harvested. We need to stick together as fedizens and defend our values, one of which is to get away from all this corporate meddling.
The creator of Mastodon has written a blog post after, from what I know, having been in talks with Meta, who required signing an NDA. And what a coincidence, it was posted on the day Threads was launched! I'm worried about the extends of that NDA.
For example, one of the things conveniently left out of this blog post is Meta's ability to analyze every single post and user going through Threads, obviously including the ones posted on Mastodon. Sure, your IP won't be visible to them, but if someone looks at your post, profile or follows you in the Threads app, what you posted is going to them. I doubt they'll be able to resist the urge to harvest all the data they can, perhaps even illegally. Threads is not available in Europe yet. I wonder why...
For the sake of completeness, if you haven't come across it yet, some of the embrace-extend-extinguish (short EEE) worries have been outlined in this post: How to Kill a Decentralised Network (such as the Fediverse)
What happens when Threads takes off and their users demand to be able to follow and be followed by fediverse users? Especially if we don't all stick together and defederate on day one. Are instance admins going to cave in? Are users going to switch to a Meta-friendly instances, or worse, Threads itself? What happens to the discourse, if algorithms once again start to encourage people to post in ways to please it?
Think of what happened, or is happening to, Reddit. Corporations, especially these monolithic ones like Meta/Facebook, don't care about their users. They don't care about open protocols. On a good day they care about giving the appearance that they do care about these things. But really, they care about their investors. About making profit and infinitely growing. We must resist this cancer on the fediverse.