In the autumn of that year an unspecified number of monkeys on Koshima were washing sweet potatoes in the sea. . . . Let us say, for argument's sake, that the number was ninety-nine and that at eleven o'clock on a Tuesday morning, one further convert was added to the fold in the usual way. But the addition of the hundredth monkey apparently carried the number across some sort of threshold, pushing it through a kind of critical mass, because by that evening almost everyone was doing it. Not only that, but the habit seems to have jumped natural barriers and to have appeared spontaneously, like glycerine crystals in sealed laboratory jars, in colonies on other islands and on the mainland in a troop at Takasakiyama.
From Wikipedia: Critical mass:
As a simple example, consider a big city. If a person stops and looks up at the sky, nothing will happen. People nearby will go on about their business. If three people stop and look up at the sky, perhaps some people will momentarily turn around, but then continue on their way. But only a small number of people is required— say, 5 to 7 (depending on such factors as the culture, time of day, width of the street, etc) — to cause others to stop and look up at the sky, too. This number is called the "critical mass" or tipping point.
Social factors influencing critical mass may involve the size, interrelatedness and level of communication in a society or one of its subcultures. Another is social stigma, or the possibility of public advocacy due to such a factor. Critical mass may be closer to majority consensus in political circles, where the most effective position is more often that held by the majority of people in society. In this sense, small changes in public consensus can bring about swift changes in political consensus, due to the majority-dependent effectiveness of certain ideas as tools of political debate.