The biggest forum style ones are https://forocoches.com/foro/forumdisplay.php?f=2 & https://www.meneame.net/ But also algorithm driven ones like Instagram and tiktok.

Spain had smaller presence on Reddit as well. They tend to keep to other spanish speaking forums, often shared with Latin America.

Basically because there are so many Spanish speakers in the world, spaniards don’t even bother to learn English or interact with the broader internet.