Oh gosh - it's been too many years since I knew all this stuff, so someone else might have to fill in the specifics...
There are several episodes - season 3, I think - where the ticker tape in the Planet has FoLC-specific messages/in jokes.
In the Tempus episodes from S4, reference is made to WAFF - Warm and Fuzzy Feelings - a FoLC fan reference deliberately used by the LNC producers as a shout-out to the fans.
In at least one episode, a statue on the table in Clark's apartment was made and sent in by a FoLC.
LabRat