The Stanford Encyclopedia of Philosophy says:
"Popper holds [that] the open society can be brought about only if it is possible for the individual citizen to evaluate critically the consequences of the implementation of government policies, which can then be abandoned or modified in the light of such critical scrutiny - in such a society, the rights of the individual to criticise administrative policies will be formally safeguarded and upheld, undesirable policies will be eliminated in a manner analogous to the elimination of falsified scientific theories, and differences between people on social policy will be resolved by critical discussion and argument rather than by force."
I maintain that the Wikipedia community is Popper's sort of "open society" for the following reasons:
1. It is possible for the individual Wikipedian evaluate critically the consequences of the implementation of Wikipedia policies.
2. Wikipedia policies can be abandoned or modified in the light of such critical scrutiny.
3. The rights of individual Wikipedians to criticise administrative policies has consistently been upheld - not only on the mailing list but also on user pages and policy talk pages.
4. Differences between Wikipedians on social policy have almost always been resolved by critical discussion and argument rather than by force.
Perhaps the only element missing is the elimination of undesirable policies in a manner analogous to the elimination of falsified scientific theories, but if so four out of five is not bad!
Uncle Ed