This may not answer your question completely, but I'll take a shot at it. I saw Eric Enge speak at the 2013 State of Search conference in Dallas. The topic was Google+ as a ranking factor. His conclusion was 'Not really'.
What about Facebook and Twitter? Google has total control of +. They don't have total control of many other social networks. A lot of social networks are too fast for Google. Their understanding is incomplete because relationships change.
How a person is connected can change in a second. Indexation takes longer than that. Perfect, up to the second, knowledge of some social networks appears to be beyond them. At least that's what I gather from Cutt's video.
The devil is in the details, social networks are a great way to get your brand/message/content out there. If something 'works' it's likely to result in organic links. But let's stop worrying about ranking factors for a moment.
The ancients in our industry existed before most of the search engines. Thus there wasn't an algorithm to worry about. They thought about link building the way it should be done. They approached link value with the question; "How much traffic can this link drive to my site, and thus convert?"
So if you have a link with a lot of visibility/traffic potential, wouldn't you like to know about it?