It is settled now: the third wave of the web is "community" aka "social network." It is imperative for any serious web player to build their community - or perish. It is imperative for any web visitor to be a part of one or more social networks - has always been the case; now, we have the means to do it.
Is today's community = yesterday's brand? Well, that discussion will have to wait another day.
In the interim, let us take a look at this excellent study on the social network building process - how do social networks form? What propels their exponential growth? Why some networks take roots while some others wither away?