Bluesky has now 15 million total users (how many active?)
Mastodon monthly active usage has dropped below 1 million.
Facebook monthly active users: >3 billion
Instagram monthly active users: >2 billion
Threads monthly active users: ~275 million
If they're cheating, they're doing a poor job of it.
You need to use non-numeric information to tell you if "number of posts" should be more meaningful than a HackerNews stranger saying "these are the vibes I get, anecdotally." Numbers are pretty appealing because we have a lot of simple and consistent tools for working with them, but we also know the number of posts on Threads are artificially gamed, because Instagram and Facebook crosspost to Threads by default for anyone who has an account.
But it's still my guess. English is my only fluent language and I don't know even 1000 people. Threads could be wildly popular and I could be blissfully unaware. Again, I'm not asking you to make any epistemologically unsound leaps.