That's a really good question, and (unfortunately) my best answer is that I don't know. I was aware that FB did what you describe, but I was unaware that it doesn't do it when you post a link via the API (at least with the link and message keys). What I do know is that when you do that manually on FB, you typically post a link as part of a message itself and FB is doing some on-the-fly parsing and doing that scraping as you type the link (you see the preview before you even post the link)--I'm not sure what (if anything) you can do through the API to accomplish the same effect.
I'll make it a task to try to figure this out, though...and if there's any API binding work that needs to be done to support it I'll add it as an improvement. In the meantime, if anyone else reading this has any insight into doing this, please let me know by posting a comment on https://jira.springsource.org/browse/SOCIALFB-91.
Spring Social Project Lead