Instagram and Snapchat seem to share a common theme: they try to solve the hurdle of photo embarrassment - they find ways to get around our self-restraint with sharing photos. Instagram does this with filters so that users can create beautiful pictures of mundane scenes. Snapchat does this with ephemeral photos that disappear .
This theme fits in with my observations of frequent users of Instagram and Snapchat. Instagram photos seems to lean towards the aspirational and idealized - an expression of a desired identity. I see my friends posting idealized pictures of food, activities, moments, scenery, and people. Filters get users to share photos of mundane moments when they would otherwise exercise self-restraint.
Snapchat seems to lean towards reality and the moment - a way of communicating your current status. I have friends that just post pictures of their faces (very specifically and almost exclusively their faces) throughout the day. Snapchat has in that sense become Twitter for photos: the disappearance of photos is not a gimmick but just a way to get users to take and share unflattering photos of themselves (the unflattering photo seems to be the hallmark of a Snapchat photo), when they would otherwise exercise self-restraint.
 The fact that Snapchat does not have any technology that will actually ensure a photo’s disappearance is largely irrelevant, for the same reason that it’s irrelevant that Instagram does not guarantee your photos will actually look good.