Each follow is a decision by a reader to keep in touch, for us to keep track of what they know and alert them when something new happens they aren’t aware of. From an editorial perspective – that’s valuable information which allows us to serve a reader better. It also lets us know exactly how many devices will be alerted when we update a story.
The follow feature is a great example of how we can measure reader engagement in the future. Mashable has a list of other useful metrics.