Once established as valuable and unique, a core interaction can come to be the reason a product scales. Look at the great digital services we have - for YouTube and Spotify, the core interaction is 'press play'. For Facebook, I'm leaning towards 'like'. (And as 'send a message' was getting close to competing with 'like' for primary value to user, Facebook cleverly spun it off into a separate app; I am surprised how many people still haven't seen Messenger.com). Both Instagram and Facebook's core interactions are supported hugely by the their common core interaction, the vertical scroll..