Framing social norms as a "social contract" is fraught in my view.
In the general analysis of norms and norm-generated obligations,
natural duties and
voluntarily assumed duties can be contrasted. Promising and contract would normally be in the second category. The obligation not to murder or rob people would generally be in the first category.
Arguably, many social norms - eg around queuing, greetings, etc - are in the first category.
And what makes social contract theory as an approach in political philosophy distinctive, and potentially controversial, is that it attempts to analyse at least some components of the first category (and in Hobbes's case, all of it) through the lens of the second category.
Here's an early explication of the term "social contract" in the context of RPGing:
The interactions, emotional connections, logistic arrangements, and expectations among the members of a role-playing group, relative to the role-playing activity. It includes both verbalized and non-verbalized components of these things.
I think comparing these to natural duties and social customs is not all that helpful, as many of them are in fact voluntary and relatively straightforwardly subject to change by way of implicit or explicit agreement: I think this is especially true for interactions, expectations and logistics.