Get one guy doing something silly, he looks crazy. Get another one, they both look crazy. Get the third guy in there and he makes that silly thing a movement! The third guy is the tipping point in this example. Neat way to think about the customers of your product. Take a look at how one guy initiates an entire dance party below:
Seth Godin sums this up well:
Initiators (the first two guys) are rare indeed, but it's scary to be the leader. Guy #3 is rare too, but it's a lot less scary and just as important. Guy #49 is irrelevant. No bravery points for being part of the mob. We need more guy #3s.