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.
I’d be happy to update it with more correct information if you can find more accurate numbers.
From https://fedipact.veganism.social/ the largest instances in order of active users are:
They are probably mean including mastodon, which is larger and generally federates with threads