Assuming equal distributions, the prevalence of each stacking should be approximately 16% with each x-last grouping adding up to 33%.
This poll (78 votes counted)
Sp/Sx - 25% : Sx/Sp - 32%
So/Sx - 11.5% : Sx/So - 16.5%
Sp/So - 11.5% : So/Sp - 2.5%
My own typings of forum members only (~120 entrees)
Sp/Sx - 16.7% : Sx/Sp - 12.0%
So/Sx - 14.3% : Sx/So - 15.1%
Sp/So - 23.0% : So/Sp - 19.0%
My typings of forum members + typings of people I know irl (~200 typings)
Sp/Sx - 19% : Sx/Sp - 14%
So/Sx - 17% : Sx/So - 14%
Sp/So - 19% : So/Sp - 17%
Forum poll distributions:
SO-last - 57%
SP-last - 29%
SX-last - 14%
Distributions from my typings of forum members:
SO-last - 29%
SP-last - 29%
SX-last - 42%
Distributions from my typings of forum members + people from irl:
SO-last - 33%
SP-last - 31%
SX-last - 36%
The forum poll shows a clear preference for soc-last typings, particularly sx/sp, which is perhaps reflecting the same trend as was noted in Enneagram Institute workshops: "It was mentioned that many people who thought they were soc-last, later find out they are soc-middle or soc-first once they actually understand the instinct." Thus the soc-last stackings are initially over-typed, when the social instinct is misunderstood.