Let's take a step back. What is a metric? It's a standard of measurement. In advertising it is designed to define a specific type of consumer behavior, be it attention-centric (clicks, impressions, etc.) or more direct response-oriented (sales, conversions, etc.).
There would be no use for new metrics if consumers on social platforms behaved as they do when on any other website. But consumers do behave differently when using social platforms and there are some key reasons why.
Most of the web operates as a one-way street , meaning that a click is an action that a consumer takes when considering only himself and his wants, needs and desires. For actions like these, direct response media measures activity on unidirectional paths. But in a social network, the user's thought process shifts to include the effects that his action will have on others who see it on the platform (liking a brand on Facebook, retweeting a post on Twitter, etc.). This "friend mindset" creates a new way of interacting with media, products, services, brands, people and culture, which leads to many multidirectional paths of consumer behavior.
Display advertising takes a mostly uniform approach to deploying media, treating all publishers more or less the same. This is the approach expressed in the IAB Ad Unit Guidelines for interactive media. With standard formats such as these, it becomes easy to group together social platforms with publishers and view them as giant standardized-ad networks.
By contrast, individual social platforms attempt to reflect a set of unique consumer behaviors. The ways that users converse with close friends, versus their interaction with the general public or even with a significant other, are considerably different. There is no value in a strategy that treats all of these environments equally from a deployment and measurement perspective. The true value lies in strategies that take advantage of the niche benefits of each social network, which requires specific metrics for each behavior.
On social platforms, every consumer action results in some form of content creation, and the spread of this content occurs at varying levels. Consequently, new kinds of content metrics emerge, intended to define the movement of this content within each channel. Metrics like resonance and stumbles become crucial in better executing a paid media strategy across social.
As consumers behave differently when using social platforms, it is not a question of whether we need new metrics or not, but assigning metrics to unique consumer behaviors across different social platforms.