When you say "social network", do you mean Twitter Clone? It seems like developers in particular have come to regard Twitter as archetypal social networks despite it's many problems and the way the user doesn't really much ability to define their own social graph. IE, N-character limit is still a determiner of Twitter (poisonous) discourse quality, just for example and Twitter isn't something most people enjoy being…
Far as I'm concerned, this is the main feature of Twitter. The only thing that really sets it apart is that every exchange is a public exchange.
This also (until recently) set a standard for statements, where if there are enough replies, and even the critical ones don't cast doubt on the statement, you could have more trust in it. Now of course it's uncertain, since Twitter automatically censors and pseudo-censors replies, including quality ones. Sometimes Twitter does this thing where it won't even show you a reply that you sent unless you have a direct link.