App Store rankings most definitely cannot be taken to be a reliable indicator for anything. Max is at No. 3, while Netflix is at No. 29 - this doesn't actually tell you anything, except that Max has more downloads at the moment.
Threads is obviously going to rank high, because the vast majority of Instagram users haven't used it yet - they're going to download it now, and so it's at No. 2. Looking at how badly the platform seems to have tanked from launch, I'm not keeping my hopes up on it being a competitor to X in any meaningful way.