Of all the different social apps that I've tried recently, Retro is the one I find most interesting: it has a very low barrier to post, encourages a small following, limits the amount of content that followers can see unless you share a key with them, and tricks users into being active over merely lurking.

It is not flashy, but it is very beautiful, and enjoy the close friends vibe, where you can see normal life (kids et al). It feels like a private photo journal, focused, delightful, no hashtags or filters.

Simply simple.