The Facebook sharing is made preferably using the og namespace tags. That works best because that way the Facebook scraper can find the information you want to have them used to share your documents. Snap! will allow you to choose the image, description, title, etc. that you want to show on Facebook (although by default this is fully automated with proper defaults.)

The documentation available on Facebook is actually very good in regard to the og namespace. There is also a full website about og and all its extensions. They have two important nodes under "bad practices":

• Do not include the website URI in the title, they anyway show the URI so you don't need to duplicate it. The title should be bare.
• Do not put a website description as a page description. Don't put any description if you do not have the time to write one. Facebook will take part of your article instead.

Source: https://developers.facebook.com/docs/graph-api/