There are two ways of 'sharing'. Awareness of the conceptual difference between them is crucial in answering the question. It is one thing, to share a car, and quite another to share a bottle of wine. As an individual, I may conclude on the basis of various interactions with you, that what I have constructed as 'society' seems compatible with what you refer to when you use that term. This conjecture may prompt me to say that we are sharing the concept - but this does not mean that we are sharing it the way we might share the use of a car.
Solipsism is the extreme idealist view that there is no world beyond our mind.
Constructivists must be unwavering agnostics with regard to 'existence' because, like Berkeley, they cannot conceive of what that word should mean outside the domain of experience. Whatever may lie beyond experience is inaccessible to human reason. Constructivism merely holds that such a reality cannot be known rationally. The construction of our experiential reality is never ad lib, but subject to constraints - and in many cases we are unable to decide whether the constraints we meet are due to inconsistencies in our construction or constitute impediments of the unknowable. The criticism of the anonymous 'commentators' seems to spring from an interpretation of Maturana's theory, and Maturana has made it clear that he does not like the term 'constructivism' with or without the qualification 'radical'.