We've talked about this before, I think - I don't think LJ has caused the decline so much, as there's just been a decline in general. The HP fandom was gargantuan once, but it was never going to be sustainable, even if the series had gone on forever. (Like you, basing this mostly off HP, as the only other fandoms I've been even slightly a part of are for long-dead series, and as such will always remain on the miniscule side.) From what I've seen, the decline's been fairly universal - journals, sites, forums, the whole nine yards. I think some big boards getting too self-important for their own good and driving people away probably helped things along, but not much more than things like the Strikethrough fiasco did.

~bragg247