Based on data from https://fedipact.veganism.social/ it seems that the majority of instances blocks threads.net. I'm sure there's Lemmy instances with either approach that have slipped through the cracks as the list is a work in progress.
The percentage of users doesn't correlate to instances as the biggest instance on the Lemmyverse has roughly 3x the users of the second largest, a NSFW instance, 4x the users of the biggest "niché" instance and 5x the users of what I see as the second largest general purpose instance.
This is the way i see the situation:
Letting meta join the fediverse means they will captivate the general audience and the fediverse will stop growing. Realizing this, it could lead a lot of contributors to the lemmy/mastodon/activitypub projects lose interest, which will slow down development and could eventually lead to the death of the fediverse project.
This is how it goes:
People get accustomed to all the content from Meta/Threads
Meta adds extra features to their website which do not work with other fediverse instances
People switch from lemmy/mastodon to threads or join threads directly and never ever consider joining the real fediverse.
The fediverse project either dies down or remains a niche project forever.