It is also about lowering the barrier to engaging with the product as well, right? Facebook has done this amazingly well with first the 'comment on this story'

It is also about lowering the barrier to engaging with the product as well, right? Facebook has done this amazingly well with first the 'comment on this story' interface, and now the like/unlike buttons. Simple AJAXy elements like these examples seem to drive up engagement and result in positive feedback for both the receiver and the provider, resulting in driving even further interaction. Facebook probably doesn't make money but it does enhance stickiness for them.