If we look at the original data rather than the summary, we can create a more useful series of pictures showing the geographic growth (and, in some areas, the lack of such growth) of the RBN. In these images, the size and colour of each ring represents the number of posts from each RBN poster in the named year.
We can see that, apart from the near-saturation geographical coverage in most of Europe and parts of the U.S. and Canada, RBN posters are scattered thinly around the world. In particular, Africa, South America, the Pacific, most of Australia, and a large swath of Asia are nearly devoid of reporting stations -- and even the stations that do exist in those places make few postings to the network (although part of this may be simply because of a relative paucity of stations heard).
So, although the growth in raw numbers in the original summary graph looks quite impressive, the distribution maps show that there is a long way to go before reasonably dense geographical coverage is in place over most of the globe. Since it is not even necessary to possess a transmitting license to establish a useful RBN receiver, it is particularly dispiriting that so many densely populated, technologically modern countries around the world have not even a single location posting received signals to the RBN.
The same data can be plotted in a less-cluttered manner that perhaps makes the lack of posting stations across much of the world even clearer: