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.
Someone posted this in another thread and the stats are genuinely misleading here. All the super large instances are federating with Meta, with like 90% of the users.
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