Neji has he's own system of seeking new Moderators, he has those wishing to apply fill out a form, then send it to him and the other Mods when he is looking, this is usually notified through the MotD when he is looking.
Unfortunately. . . with games like this where the demographic isn't extremely wide, the people will eventually grow out of it.
As such, it only makes sense to switch out the moderators continuously, say every three-four years.*
Unless of course, they are active, which was touched upon in the OP.
I am not to sure why Neji hasn't done any switches with the mods, just kept on adding them on.
*Most of the players on SL, Staff included, that was involved at least three years ago are no longer active. Claimed as "Retired" or have completely deleted their chars. Of course, this also has exceptions, Myself for example. (Even though I left a few times)