A couple of River SUP specific resources on Facebook. Feel free to add to the list.

River SUP (public group). Started by myself this month, as a resource for general river SUP photos etc (doesn't have to be hardcore):
www.facebook.com/groups/riverstandup/

Whitewater SUP (public group):
www.facebook.com/groups/305858769510089/

Riversurf Community (closed group). For people who surf on rivers, mostly without a paddle:
www.facebook.com/groups/185862384802650/

Limmat SUP (public group). A local group in Zurich, Switzerland, for coordinating spontaneous river SUP sessions:
www.facebook.com/groups/limmatsup/