Your contacts can click into the "Share this email" section in the footer of your mailing, then click on the Facebook icon to get started. The post will include a screengrab of your mailing, your subject line, and the preheader/plaintext of your mailing -- also, your contacts can add a personal note along with the post. You can take a look at this Resource Center article for the full run-down on our social sharing feature and Facebook.
If that's not what you're after, can you clarify what you mean by sharing so "the top image and subject line are the only items people see"?
I'll keep an eye out for any further information you might provide here!