As I just noted on my own LJ, I never assumed that Cath had no fandom friends, simply that because her interactions with them would've been strictly online, we just never heard about them because Rowell seems to have made the decision to exclude all that from the narrative. It makes sense that if a person with an active fandom life was writing the story, they would have noted that Cath opened her chat option while she was writing or checked her email constantly and had various conversations as it was going on, etc.--but I think you're right, Cath's behaviour is more like that of a lurker, which (from her own epilogue) I kind of assume Rowell was, during her "research" period. (I just made the decision to include various online friendships I have in Experimental Film, because it'll make it sound more like my actual life, but I can totally understand why she might assume doing it in Fangirl could alienate the non-fannish portion of the audience.)