Most of the answers here mention hardcoding campaignSource and campaignMedium.
If you do this, your analytics will miss recording any specific campaigns using UTM tags.
Also, Google Analytics will treat all your traffic to Instant Article as Campaigns and numbers won't come under "Social" Traffic.
The solution is to set the referrer to https://i.facebook.com/.
Using this Google Analytics will treat your traffic is coming from Facebook and behave as if it is "Social" traffic.
To distinguish Instant Article Traffic from regular Facebook traffic you can filter on referrer or add custom dimensions.
Do remember to change XX-X and in the following snippet.