This poll is quite odd, because I remember seeing polls in model railroader with results showing the gap between HO and the other scales being much, much greater. I thought HO was 70%+, N was 16%, O was 8%, etc...
I model in both HO and On3. For those that know of On30, but not On3, On30 is 30" guage trains in O scale...On3 is 36" gauge trains in O scale. 36" gauge is much more accurate for american prototypes (such as Bachmann's mogul, BLI's consolidation), but it generally is more of a craftsman gauge due to the a lack of RTR equipment and difficult to find flex track (no sectional track available). Fortunately, the influx of high quality On30 models has given us a good supply of cheap equipment that just needs to be regauged by 3/32" and it increases the interest in narrow gauge modelling.
I used to model in N, G, and HOn3. I still occastionally dabble in HOn3.