Another now-classic example of solving this chicken and egg problem, I think: Facebook starting with just Harvard, then ivy league colleges, then colleges, then

Another now-classic example of solving this chicken and egg problem, I think: Facebook starting with just Harvard, then ivy league colleges, then colleges, then colleges and high schools, and only finally the general public. At each stage the arena was small enough it was realistic to take the whole segment over, and then that became the seed for the next phase.