My impression is that the author is analyzing the successful social networks. Think about all the ones that died in the process of Facebook/Instagram/Twitter, etc getting to where they are? All of those lacked ways to "building social capital through signaling".

I might be wrong though, didn't put a lot of thought on it, but friendster, orkut, myspace, and many others were not quite as good in signaling or proof of x as the ones that are now well stablished.