<meta> description should be blog description for non-posts.
Add Twitter card/Open Graph <meta> tags.

See https://dev.twitter.com/docs/cards/getting-started and https://dev.twitter.com/docs/cards/types/summary-card. Inspired by http://timheuer.com/blog/archive/2013/11/03/adding-twitter-summary-cards-to-blog-content.aspx.