I did a poll and nearly 30% of the people who voted said they used Sync! It's stayed around that percent the whole time too!
EDIT: Here is the poll I forgot to put this here like five times lol https://strawpoll.com/wby5A21R1yA
lemmy.world has 3 times the amount of users as lemmy.ml. Also lemmy.ml has an application process for becoming a registered user, so there definitely is some kind of bias as to who is registered there.