Madrigal explores it:
1. The sharing you see on sites like Facebook and Twitter is the tip of the 'social' iceberg. We are impressed by its scale because it's easy to measure.
2. But most sharing is done via dark social means like email and IM that are difficult to measure.
3. According to new data on many media sites, 69% of social referrals came from dark social. 20% came from Facebook.
The Dish 