ashtophoenix's Journal: Gaining a Critical Mass for a Network Effect

As more and more developers/entrepreneurs are starting up social networks, either focused towards a domain or general, it seems to me that the biggest obstacle is to gain a critical mass of users before you start advertising your network to the general public. How do such networking (social or otherwise) applications gain such a critical mass of users? For example, if you are building a social networking site for doctors, how do I get a critical mass of doctors to sign up such that those joining hereafter have enough benefit from the existing user-base that they want to be a part of the network.