Someone with thousands of online followers may feel their circle of friends is all but limitless. Evolutionary anthropologists offer a reason to doubt that impression. On the social brain hypothesis, what pushed the human neocortex to grow so large was not the challenge of making tools or solving problems in the physical world, but the sheer difficulty of living among other people. Holding on to a relationship takes steady mental work: recalling what passed between you before, tracking who now stands with whom, guessing what each person will do next.

Since the brainpower for this work is limited, the theory proposes a ceiling on how many such bonds a person can carry at once — a number usually put near 150. Social platforms let us reach far more people than that with almost no effort, but reach is not the same as capacity, and no amount of new technology rebuilds the old brain behind it. For all the machinery built to connect us without end, then,       ?      .