Using Facebook Graph API, for group posts, we were able to retrieve the data for comments and likes. Now with the introducing of Seen By info on group posts, there seems no API exposed to get the Seen By data yet, or there is? We are not able to find that info with documentation at https://developers.facebook.com/docs/reference/api/post/.
As @Tommy Cursh suggested, currently there is no API available to get the Seen By data programmingly. Hopefully Facebook will provide it someday soon.
来源：https://stackoverflow.com/questions/13667448/how-to-get-the-seen-by-data-for-group-post-on-facebook