I see similar too. Both my teenagers got WhatsApp because we as parents had WhatsApp. They have slowly started using Signal in their friends groups. Now as a family we use Signal because the kids started us on it. We are based in Europe and iMessage is almost never used. I’m only on WhatsApp now because other parents are still using it. Sadly my oldest uses Instagram (on a strict daily timer), but apparently “it’s still cool to have an insta” and the killer feature there is that is super easy to network without sharing your phone number (I know signal also has this feature but it’s a bit hidden).