I agree it may be in its infancy but so many of the startups I see (among the ones that fail too) are all about sharing and nothing else. A joke that turned into viable idea[1] is emblematic. Foursquare started like you say, but I'd say it's not used by the majority of people to meet with friends. Instagram pivoted to follow themselves this trend. It doesn't answer to a need of the founder so it is not an organic startup if you consider pg's definition "those [startups ideas] that grow organically out of your own life"

I am not arguing that sharing is not popular. I am saying that it's and ephemeral trend.

[1](http://www.jotly.co/)