A good question! And I have no answer. The two best known methods are premium accounts – the members have to pay for their account – or simply advertising.
The first one harbours the risk, that one day a competitor offers the same functions for free the other communiy offers with their premium account. This is now the case with myCORNERS, a business network, which offers a lot search functions you have to pay for within Xing.
The second method, advertising, can be also a big problem as seen with Facebook and Xing a few weeks ago. Facebook wants to places ads depending on the profiles of their members. This causes at the end of 2007 an outcry within their community. The same with Xing: they wanted to show the same ads to their premium members like they show to their basis members. This causes also trouble with their community. At least Xing withdraw their plan.
As I wrote in my article What is Web 2.0 (sorry, in German only), business models based on social communities are very dangerous. The coupling between the provider and the customers is quite loose. If the provider annoys the members to much or the customers can get cool functions for free, they simply switch to another platform. The others are just a click away.
We’ll see how things evolve.