Well, not LJ, but fandom in general. I saw the first HP movie, read the first book and tossed it aside as mediocre writing and childish characterization. Saw the next two movies because, well, they looked good, and they were. Didn't touch the books.
Then, last summer, I got into metaquotes for some reason, and they linked to a lot of fanficrants, which amused me from time to time. I also noticed a few icons that were funny and had the word "sbp" in them. Then mctabby's "summary executions" got metaquoted, and I knew enough about the relationships between the characters to find many of these extremely funny. Then up came perposterice, based on those "summary executions," and I read along and laughed. I also saw more icons with the "sbp" bits in them. I looked into "sbp" a bit and found shoebox_project. I read Chapter 23, the newest chapter at the time -- which, if you haven't read it, is The Kiss(es). The moment the authors had been working toward for over a year.
I said, I must read this.
So I did. I think it was August. And as I read it, I fell in love with both Remus Lupin and Sirius Black. I turned to my sister (who had given up on trying to persuade me to read the Harry Potter books) and asked a million questions about Remus, Sirius, and the whole thing. I was working from practically a zero background, here. I knew next to nothing about the Marauders, Snape's worst memory, the Shrieking Shack incident... after all, it had been a while since I saw the Prisoner of Azkaban movie, and I only saw it once!
Anyway, my sister said "you should at least read PoA." I picked it up, and noticed it had a chapter titled "Moony, Wormtail, Padfoot and Prongs," so I turned straight to it and read it. Happy sigh (though part of me said "THIS is all we have to go on?!"). This was about last November, and I was in a prime position to read a lot of books, so I read Prisoner of Azkaban. Then I saw the Goblet of Fire movie (and, for some reason, thanks to shoebox_project, my eyes were on Snape the entire time he was onscreen. Movie!Snape is THE SEX).
Then I gobbled up Goblet of Fire, Order of the Phoenix, and Half-Blood Prince in about four days (I think it was Christmas when I finished).
Ahhh, I said. And, hmm.
Then I turned back to LJ, and used fanficrants to find "The Pit" (www.fanfiction.net). I was so brand new to the world of fandom it was painful. I still don't know what a lot of terms and abbreviations mean, but I am learning.
I think I have arrived here in a very, very odd way -- I would never have read the books if it wasn't for fanfiction -- but I think it gives me a good perspective. I bring fresh eyes, in a way. Of course, that means I am stumbling in places where others have stumbled without the benefit of their knowledge, but I'm playing catch-up as fast as I can, thanks to hp_essays and a few other communities.