My exposure to fanfiction ran roughly as follows: randomly stumble across Cassandra Claire's "Very Secret Diaries" --> Cassandra Claire's Draco Trilogy on FictionAlley --> other stories on FA plus various Yahoo groups --> SugarQuill and Gryffindor Tower --> start writing my own HP fic --> fanfiction.net (back when porn was still allowed! oh god now I feel old...) --> Livejournal via isiscolo's LJ invite code project --> start reading crossovers on ff.net --> discovery of Naruto largely via Asuka --> start writing my own Naruto fic --> slow realization that I am never getting out of fandom alive (not that I want to get out, but you know what I mean).

But HP fandom was kind of a law unto itself in those days so I would not use myself as a good case to generalize from. :-)