We use facebook to promote our forums with members. When we share some forum link with members, the Facebook page is unable to draw a image from attachments and create a thumbnail from it. The thumbnail image is important because it tells people something about the link and draws their attention towards it. Consequently the impressions are higher from a link with a thumbnail which relates to the topic of the link.
Our facebook pages are able to draw images from a vBulletin forum of ours but it is not able to draw images from a Xenforo forum and create thumbnails from it. We just get the members avatars which usually does not relate to the thread topic.
I request you to please look into this issue.