Postcrossing covers the entire globe but is anyone a member of a postcard sending group within their own country?
I started my own little project in the UK and have around 25 people exchanging cards on a monthly basis. It’s nice to receive cards from around Britain as I only get the odd one now and again via Postcrossing.
I’m fairly sure on this forum there are several round robin groups that are just within a certain country, if that is what you mean?
Totally, just have a look at #communities:british-isles and all other #communities. Not to count all those groups on Social Media, @BrianFraser 
Yeah, there are a ton of Facebook Postcrossing groups within countries - we have at least 2 in Canada.
I am technically in Nordic and Finnish postcrossers groups in Facebook, but I never write anything there or join the swaps. I am just lurking there for fun
The Nordic postcrossers group is actually much more active in Facebook than here on forum.
Prior to joining PostCrossing, I belonged to several US-based groups on Yahoo. Within a short time, I left every one of them. Only do Postcrossing now.
I became a member for quite sometime but wasn’t really active until recently
I think there are some local facebook postcard groups as well.