Current best answer seem to be Mastodon and Bluesky.
If we are going by the number of users, Facebook has more users than Twitter and Threads has about the same order of magnitude, but we still see a lot more Twitter submissions to HN compared to Facebook and Threads[1].
If users prefer the microblog format, Mastodon and Bluesky appear to be the most compelling alternatives, both have been steadily gaining users[2].
[1] Submissions from various sites:
https://news.ycombinator.com/from?site=twitter.com
https://news.ycombinator.com/from?site=threads.net
https://news.ycombinator.com/from?site=facebook.com
https://news.ycombinator.com/from?site=bsky.app
Not sure how we can find the number of Mastodon links since it has so many domains.
[2] Mastodon and Bluesky user counts:
https://mastodon.social/@mastodonusercount/11213120632905851...
https://bsky.app/profile/bsky.app/post/3klzrudt4uk2z
https://bsky.app/profile/bsky.app/post/3kkv3clm3su2h
