I'm a little unsure what you mean by referencing Digg and Reddit, however both of them were/are limited to the tech field. They both started as programmer link

I'm a little unsure what you mean by referencing Digg and Reddit, however both of them were/are limited to the tech field. They both started as programmer link sites, but that side of the sites withered away as they became yet another mainstream site. LinkedIn was dominated by the tech field. Specifically the valley tech field. That was what gave it the network effect.