Is it possible to determine which Facebook "post" a visit comes from, via the V2 campaign tagging (at_medium)?
Because the source of a Facebook "posts" comes from a third-party site, we cannot automatically retrieve it.
To help determine which "post" a visit comes from, we recommend that you use a marketing tracking link. One of the variables of this link will allow you to distinguish the Facebook "post".
One of these variables would then be based on a custom property, created upstream.
You will find documentation on the creation of a custom property by following this link: Properties.
Smarttag tagging above 5.29 and Piano Analytics SDK
The property_key of this new custom property should start with the "src_" prefix in your Data Model. Any querystring parameter beginning withat_
will be considered a campaign parameter, and will be translated into asrc_
property
Example (Data Model - Creating a Custom Property):

In the case of Smarttag usage, custom properties will only be accepted if the campaign source is also custom. If a native source is used, you can use the native parameters (Smarttag documentation : https://developers.atinternet-solutions.com/as2-tagging-en/javascript-en/campaigns-javascript-en/marketing-campaigns-v2/ ).
https://www.site.com/at_medium=Facebook&at_campaign=my_campaign&at_facebook_post=my_post
You can find more information on tagging marketing campaigns via this link: https://developers.atinternet-solutions.com/piano-analytics/data-collection/how-to-send-events/marketing-campaigns.