Phoca Open Graph uses the following process to display description in open graph meta tags (og:description):
1) Description which is set in parameters of the plugin (plugin parameters) will be set for the description meta tag on the website.
2) If no description is set in plugin parameters then article meta description is used.
3) If no article meta description is set then menu link meta description (menu link to article) is used.
4) If no menu link meta description is set then the article introtext is set
5) If no article introtext is set then the site meta description set in Joomla! Global Configuration is used.
Don't forget that this plugin is a content plugin and it can render the meta tags only on article sites (com_content). In article view or in blog view where more then one article are rendered).
This flow was voted by users and it can be easily changed in the code (plugins/content/phocaopengraph/phocaopengraph.php).