Social constructionism focuses on objects that are created through interaction between people and so attitudes towards those objects are formed. A good example of a social construction is ideas of what we find to be immoral, for example polygamy is illegal in the UK but in some religions/cultures marrying multiple people is socially acceptable. The concept of polygamy being immoral is a social construction, as is polygamy itself.
Social constructivism on the other hand focuses on individual's learning of objects through social interaction, for example how a person's attitudes and understandings are formed through discussing a topic with other people. This is a theory of knowledge rather than interaction, and focuses more on the individual than the group.
Constructionism = physical social interaction
Constructivism = cognitive process of learning
I'm no expert on the topic but this is my understanding, if anyone else has a different explanation feel free to correct me