Was Piaget a social constructivist?

To Piaget, all "knowledge" is constructed. His theory is meant to apply to all cultures in the same way, in that they develop the same knowledge structures (so knowledge itself isn't the product of culture, but its development is helped by parents, teachers and peers). This can be contrasted to Vygotsky who sees cognitive "tools" that children develop as the product of culture first, then transmitted to the child.

Theres a really good book by Piaget's colloborators who begin to experimentally study the effect of teaching on cognition, and in the introduction they talk about the relationship between learning and cognition in Piaget's theory. Its called "Learning and the development of cognition" by INHELDER. The first chapter is really easy to read (compared to Piaget, who is really hard to read) and sums up the theory!