Manual Excerpts

Must I enter an excerpt on every post with Infinite SEO? When I fail to manually create an excerpt, the Google+ button and the Facebook button won’t get anything from the WordPress excerpt. Google+ gets the first text on the page and Facebook pulls nothing. If I manually enter the meta description then will it work? Why can’t the meta be populated by the first lines of the post as a default? Or is something else going on?

The problem occurs on most posts on using either Facebook Share from AddThis. This all worked fine when I was using a different SEO plugin.