As I said in another comment, all of these statements are true. Blue Sky blew up very quickly and starter parks were a key part of that. Blue Sky is also losing users which I find to be expected. Threads is quickly gaining users because of network effects. If you have an Instagram account, you have a Threads account.
How has threads managed to gain 275m users while producing absolutely zero impact on any other part of society? Are you sure these are actual human beings and not just Instagram users who are being baited into clicking through and "signing up" for a service they don't use?
Am I sure they aren't? No, in fact I think that is exactly what they are. Or people that made an account out of curiosity because it is so easy to do so. However, that delves into speculation and we won't get a full answer on that because Meta will never say.
These facts are related. "starter packs" only work if you're cultivating a very narrow social bubble of users, which is the same reason bluesky is declining now.
Unbubbled people have a wide range of interests and preferences that cannot be captured in a few "starter packs".