The network effect, or, demand-side economies of scale.
This describes a circumstance wherein the value of a service or product is related to the number of people having adopted it (or something interoperable). Total effect refers to the value added to other users when adding a user, while marginal effect refers to the increased motivation to join for non-users.
This is different than economies of scale, where higher production means decreased production costs. It is the demand-side equivalent, however.[^rWP]