The discussion about the difference between social constructionism and constructivism has been going on for a while here.  There is a simple way I keep these 2 schools of thought apart in my mind, although I think they offer a lot to each other. 

In Maturana and Varela's constructivist writings,  they position the reader in 
one of 2 stances:  We are either "outside" the organism [1] in the community of 
observers making observational conversation with each other about the 
organism under study ; or we are 'inside' the organism describing what's going on (Maturana & Varela, 1980).   Varela talks about two types of explanation symbolic explanations (which is talk about what is going on between organisms - either with each other or their environment -  by a community of observers); and operational explanations (which is talk about the internal processes of an organism which maintain it's autonomous identity) (Varela, 1979).

An organism in Maturana and Varela's (1987) autopoietic constructivism is informationally closed in the sense that a kind of solipsism exists for the organism - e.g. the rods and cones of the eyes are perturbed by light and as a result it's nervous system constructs a world. To the degree that we don't bump into doors etc, we are constructing a world that 'fits' with the world being 
constructed by the community of observers. 

When Maturana & Varela rose to popularity in family therapy circles in the mid 1980s, it was largely because they seemed to be providing a theoretical foundation for biological and social sciences which could be as secure as that found in physics and chemistry.  For their new science (autopoiesis) they said we need to engage in both types of explanation, but not to mix them.  But I think many family therapists discovered that providing explanations; either about what was occurring internally to the organism, or between the organism and it's environment, was not a useful therapeutic program.  We needed to find a way of being at this point of interaction between these two modes of explanation, outside and inside the organism. 

By contrast Gergen's social constructionism focuses upon relationship - upon 
the boundary if you like between 'inside' and 'outside'.  If I think of a 
simple gestalt visual puzzle such as the 'kissing vase' or the 'rabbit 
duck' - I might say that Maurana and Varela's thinking would have me focus on one or the other images whereas Gergen would have me focus on the actual lines between them. 

As a therapist I find Gergen's system more respectful and useful as it allows the paralogy of 'not knowing'  conversation (whether it is a 'duck' or a 'rabbit') to flow. 

1. I am using the word 'organism' as their work was primarily aimed at biology in general 

2. Varela, F.J. (1979). Principles of Biological Autonomy. N.Y.:Elsevier. 

3. Maturana, H.R., & Varela, F.J. (1987). The Tree of Knowledge. 
London:Shambhala. 

4. Gergen, K.    When relationships generate realities: Therapeutic 
communication reconsidered. Draft copy for edited volume by Eero Riikonen. 
http://www.swarthmore.edu/SocSci/kgergen1/text6.html