I would like to recommend Geoffrey Campbell to the math-fun mailing list. His Facebook profile is here: https://www.facebook.com/geoffrey.campbell.180 He says he has attempted to subscribe via https://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun but "It kept declining my email address and proposed password - the entry requirement." At any rate, Geoffrey regularly posts some interesting math puzzlers on LinkedIn Number Theory group, which is how I encountered him.