Starting a test where posts from Threads accounts will be available on Mastodon and other services that use the ActivityPub protocol. Making Threads interoperable will give people more choice over how they interact and it will help content reach more people. I'm pretty optimistic about this.
I have my problems with Meta, but I'm hoping this will help Mastodon grow
If Meta can read things you post to Lemmy? They already can if they're so inclined; it's all indexable by anybody with a web crawler bot; robots.txt on lemmy.world doesn't even discourage it. If people can post to your favorite Lemmy communities from Threads? Don't expect many people to do that - there's enough UX mismatch it's an awkward experience from any microblogging software.