I Like number 3. I think there's two ways this works: 1) Killer content is created and social shares quickly give signals of freshness and quality within an area. Combine that with early user metrics as the content moves up and you have a good idea of it's quality. However, if this content is not soon backed with corresponding signals (links, etc.) the ranking can fade. This is likely dependent on the industry and topic as to how it plays out.
The other possibility is a good piece of content earns links, has great user metrics for certain keywords, lives on a great site, etc. and ranks well. It then accumulates social shares on top of that, further confirming the quality of the content. Thus...you get a situation like #3. Social signals make sense for indexation, but beyond that, until you start to understand the importance of the author (cough, AuthorRank, cough) then you can't use them for much more then understanding what the "mob" is finding valuable at the moment. And it might be a very fleeting fascination on a subject that really doesn't deserve to hold a ranking.