Using metrics to build communities
There is a Branch called Do platforms need to give users a number to optimize?
Most platforms do provide a number to optimize: Twitter followers, Facebook friends and so on. These metrics build liquidity in the platform as @satyap, @ev and @hunterwalk point out.
But there isn’t just one type of social metric. There are three. It’s important to distinguish the types of metrics a community can provide for users to optimize, because the community will rally around the metrics the community provides. The long term dynamics may not be the expected or desired behavior.