There have been a number of debates about he Validity of Social networks and their true value. danah boyd is one of my favorite reads and she has a consistent habit of repeating the need for people to examine the SNS issue with context in mind. Context is what we use to define a relationship in real life and I am sure that she would love to see context somehow worked in to the online version. So would I.
From a business point of view it allows the users to more effectively leverage relationships for prosperous ends. How can everyone be your friend? Ze Frank made an absolutely hilarious short based on his SNS gripes. He brings up numerous valid points interwoven with his own brand of satire comedy.
So in a nutshell context can be immensely valuable in community building. If you are able to see how people know each other and why they are connected it can allow members to effectively communicate and remove the awkward burden of asking a total stranger for a favor through a relatively unknown acquaintance.