I'll answer your questions the best that I can.

I do write one other fandom, but have never posted anything online.

Mulder/Krycek got me into slash. Before that I read Mulder/Scully.

As for leaving a fandom, I'm more a reader than a writer but I don't read X-Files fanfiction anymore. Now its just Harry Potter and usually only Harry/Snape.

I think the Harry Potter fandom will go on for a long while. It seems like the communities I have joined are constantly getting new members.