Chartbeat have been diving deep on the data and the answer is…

Facebook shares and reading time

While there is a small positive correlation between shares and total engagement, the relationship between the two is quite weak. For stories that attracted 1000 shares on Facebook, the Total Engaged Time they earned ranges from around 14 hours to over 1000 days. This tells us that social media interaction and actual reader engagement are not as closely aligned as many tend to think.

…sometimes.

It’s clear that just tracking Likes and Shares is not enough to understand the impact of social on your site. You need to related them to reading time, too.