[account42]

A social contract isn't a legal contract to begin with, but even for those "written or explicitly spoken" is not a hard requirement.

[skeledrew]

A social contract still has to be explicit in some way to be considered such. Otherwise it's just an accepted convention.

[jhbadger]

That's not what a social contract is you are thinking of a legal contract, something very different. A social contract is by definition, implicit rather than explicit.