A recent study shows that dark social matters more than you think.
Dark social is all the hidden social activity that isn´t possible to track. For example if you found an interesting article, but don’t want to share it with your entire network, you’ll send it in an email or private messages to a friend rather than sharing it on social networks.
Read more about Dark Social and how Google Analytics tracks the visits of your site from content that is shared in for example emails.
You can read more here (In Danish)