Twitter has recently been trying - hard - to re-enage me with the pitch 'Go back to basics.'
Besides the poor visual design that invites pressing the 'trash this' button, notice how the message is focussed on getting me to perform just one single interaction - tapping the heart icon to like a photo.
My point is, even unicorn league products find it incredibly hard to engage users beyond the core behaviour that the product is built around, in this case the 140 character tweet. Tapping a heart dead centre on the screen to like a photo / video is an interaction that Instagram already owns in my mind.
Twitter should be doing something to engage me by extending or adapting the core interaction it already owns in my mind. Thats Twitter's greatest asset.
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..
Scaling a product beyond it's core interaction should be a matter of deep consideration and understanding of the behaviours they create and own in the user's mind.