Instagram started because Kevin Systrom's current app was mostly being used to broadcast photos so he pivoted. He also had experience in photography which led to the photo filters being a key addition to the product.
I don't know enough about Vine to comment, but Foursquare and Instagram both provide a ton of consumer value. Based on how they tell the stories of their origins, it seems they were also "organic."
Lastly, social is not a trend; in fact I believe it is in its infancy.