You have seen graphs before in Books 1 and 2 as data structures and algorithm inputs: adjacency lists, shortest path algorithms, topological sorting, minimum spanning trees. You have seen them in Books 3 and 4 as infrastructure topologies: service meshes, replication graphs, distributed hash tables. In each case, the graph was a representation of relationships that the system needed to traverse or maintain. This chapter examines the graph from the outside: the product itself is a graph of users and their connections. The engineering implications follow from its growth properties — as nodes are added, the number of potential connections grows as the square of the node count, and architecture must anticipate that growth before it arrives.