Hard to quantify and possibly more open to abuse, but an approach from this angle would be interesting to see.

A lot of data points and would require profiling behaviours which means some behavioural psychologists, and behavioural economists and the like to build. Just like FB use.

Yup, and perhaps not one solution will fit all SMT

I think they will experiment based on their target market and see if they are able to drive behaviours that support their particular platform.