Another way to think of the difference between snapshots intended to reach across time, and snapshots intended to reach across social connections, is this.
In the film era, the snapshot was supposed to reach across time. It was a tool to trigger memory, "Remember that trip to Disneyland?" and as such is filled with temporal and spatial references. The pictures from Disneyland contain The Mouse, and The Castle. They are recognizable, more or less instantly, as "us, in Disneyland." They would make no sense without the specific objects which tie the photograph to that event, that time.
Pictures from Christmas inevitably contain the Tree, birthdays have Cake, and so on.
These days we get a lot, a LOT, of photographs with no such things. A picture of a latte is useless as a spur to memory, I've drunk a lot of lattes in my life. A picture of me at a party is useless as a spur to memory, I've been to a lot of parties.
As a tool to tell my friends "I am drinking a latte right now, and it is delicious" a picture of a latte is marvelous, however. It neatly encapsulates the moment in an instantly apprehensible object.
In this light, I am being very unfair to the photographic-safari people. These people are generally taking pictures that are rife with objects that anchor the photo to a place and time. They are excellent spurs to memory, in general, and as such could well serve to reach across time to a future self. And, indeed, I am certain they do serve this function. The wealthy gents who take these things, no doubt haul them out 10 years hence and recall their trip to Africa.
Still, I continue to hold that the primary function of the pictures is social signaling in the here-and-now. The pictures exist to say "nice latte, but I was in Africa."
Sally Mann argues that photographs destroy memory, and I am inclined to agree. But she is referring, I think, to the photos that we use as spurs to memory. The photos from Disneyland become our memory of Disneyland, the photos of Dad become our memory of Dad. I'm not sure what the photos of lunch, parties, and lattes do. I already don't remember those things specifically. I think we generally remember those things mostly as categories rather than specific instances.
Does the line between the category of things, and the category of photographs of the things, get blurred? If you're in the habit of photographing your latte, when is your idea of "a latte"? Do you think of an actual latte, or is what floats to mind an instagram photo of a latte? I don't habitually photograph my drinks, but maybe I should start.