Thank you all for your replies. I studied Social Sharing core component
source code for a bit and ended up re-using getThumbnail() method to get
page thumbnail reference and that worked out as expected.
Yes, seems like it's coming from DAM
However, I am not able to find fileReference property anywhere under the
image node or under any node.
Is it possible to insert thumbnail image URL from page properties of
Thumbnail tab into og:image inside meta tag. I am able to get values of
pageTitle and description properties but not the image. Or is there a
better way to handle this?