Context matters. People landing to your blog post about turtles are in a different mindset if they are coming from a Facebook Ad, a LinkedIn article, or have searched about “turtles”.
They might belong to the same group: millennials between 25 and 35. But the emotional state will be different depending on what they saw one click ago.
Google Analytics can give you a high-level overview of average session time depending on the referral channels.
On this case, people who came from social channels spent more time (on average) on the website.
Does this mean you should spend more time on social media? Not really. It only means that people who came from social media spent more time. Next task for you is to figure it out why: what was the message, device, location; in which blog post they spent more time as a group.
The intersection between audience and content that led to that boost in time spent is what you need to find out.
