Shields up, folks! 
Facebook is joining the Fediverse and you know what that means. Adtech, surveillance, privacy-invasion-as-a-business-model, and all that. It's what they do. It's what they've always done. We don't expect this time to turn out any differently. 
The most effective way (that I know of) to fight back is to completely block any Facebook-related IPs at the firewall level. This will prevent Facebook instances from federating with your instance. It will also prevent Facebook scrapers from trying to harvest information from your instance's users, which Mastodon's defederation tool can't do.
I made some simple tools to help make blocking Facebook easy for instance admins:
codeberg.org/cuchazinteractive…
In this repo, you'll find the ASNs known to be used by Facebook and lists of current IP address ranges for those ASNs. There's also a python script to turn those IP address ranges into firewall rules for iptables, the firewall used by the Mastodon instance setup instructions. And there's a short readme to explain how to use the script and download the IP address ranges.
I've confirmed that these IP address ranges will block threads.net, and threads.instagram.com, which are domains currently known to be used by Facebook/Instagram instances.
Happy blocking, folks! Stay safe out there.
