There are two kinds of PPRuNe reader - the registered kind that log-in and visitors that are read only.
As Tight Slot referred to above, there is also the fact that one used to be able to read in at leisure without having to log in. Now (since a couple of months ago i believe?) you are only able to read a few posts before the requirement to log in comes up. I'm pretty sure that must also have an effect on the number of new members?