This excellent post by Danny Brown makes a great argument for why the endless pursuit of eyeballs is a waste of time.
Yes, advertisers buy ads on the superbowl to reach a large audience but social media is the opposite of the superbowl. It is, or can be, a conversation between and organization and its stakeholders. Its very nature suggests that your social media goals should not be "reaching as many eyeballs as possible" but rather reaching the right eyeballs who have a problem that you can solve.
But that isn't an argument I'm going to solve today.. which is why we have a broad coalition of advertisers, associations and pratictioners who have been working for a year and a half on establishing standards. You can read all about that effort here. And the latest standard definitions for reach, impressions and engagement are available at the SMMStandards Page. I encourage everyone to go read them.