I think when the fandom was new, more people were joining in the fandoms and that was when the message boards took off.
Around the third book, fandom was well established on LJ and they also had free accounts.
As the fandom took shape,people started looking for the part they felt was interesting or enjoyed participating in, that is when journals took over from message boards in a way. IMO the younger fans still frequent the message boards.
A total newbie will not be able to find anything much on LJ. They hit the boards and find stories recced on LJ, and slowly they settle down to interatcing with others on LJ as they get to know them.