In its initial stages, Facebook had limited value to users due to the lack of network effects and critical mass. The principle behind the strategy is that at each time Facebook enlarged the size of the community, the saturation never drops below the critical mass, reaching the desired diffusion effect discussed in Rogers' Diffusion of innovations.
At first, Facebook did not offer much utility to its users due to a lack of network effects and critical mass. The approach hinges on the fact that as Facebook grew the community size, saturation never dropped below the critical mass, leading to the diffusion effect mentioned in Rogers' Diffusion of Innovations.