My first fandom was The X-Files. Though I don't participate since the series ended, I know the fandom is still alive and well as I still get an email now and then about stories I've written for TXF. This is the third fandom I've written for, unless you count the limericks, because then I've done LotR, PotC, QL, Buffy, Totally Spies, X-Men, Star Trek and probably some other things I've already forgotten about. But as for fics, Juxtapose Fantasy, TXF, and HP. :)

I should think most book series fandoms could outlast television ones easily, and I would foresee the HP fandom surviving well past the conclusion of the series, both in print and film.

As a side note, what is TPM?