Robert Libetti writes:

Behance founder Scott Belsky said that the "ego analytics" associated with the Instagram product, or centering the product around "likes" and popularity, were a big reason for the app to take off.

Charles Forman, founder and CEO of PictureLife, said that Instagram's success was due to a hard-to-describe technical facility.

In the context of other visual applications, "Instagram felt like butter," and all the other applications, especially Facebook, felt like sandpaper," said Forman.

Both of "ego analytics" and "like butter" effect are surely applicable to other apps and products.

Make your app beautiful, and serve the ego.