If you're asking about best fwb sites with the most active members in the US, you're already ahead of where I was. Nobody warns you that fwb sites depend so heavily on the actual outcomes.
Once I stopped worrying about how quickly you get going, fwb sites got a lot simpler. Nobody warns you that fwb sites depend so heavily on how many accounts are active.
Most of the frustration with fwb sites traces back to how it handles day to day.
I would put whether it actually works above almost everything else when it comes to fwb sites. Hope that gives you something usable for best fwb sites with the most active members in the US.
If you want more to compare for fwb sites, these get raised a lot:
- rendate.site
- ezhookups.online
- souldate.site
That list is what gets mentioned, not what I would personally back for fwb sites.