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.