Google classifies traffic as Social when it comes from approximately 400 networks (that is not targeted as an Ad).
The list of social networks has not been made public by Google, but there are some people has been trying to figure it out, if you are interested.
Google Analytics filters traffic to this channel when:
- Social Source Referral exactly matches Yes OR
- Medium matches regex ^(social|social-network|social-media|sm|social network|social media)$