They're the exception rather than the rule. The majority of the time you won't get any value from a visitor unless they complete an action while they're there. That's a bounce.
blog.yourcompany.com isn't doing it's job unless it engages someone enough to share the content / comment / find out more about the product and on...
StackOverflow have established dominance in organic search and buckets of engaged users: not many sites are in the same position.
Track any other action a user takes in-page like sharing/commenting with a GA event.
