The user ID is used to create a fb:admins meta tag, which Facebook uses to determine who can do things like view insight information for your page. This tag will not be created until/unless you set the user ID. Other meta tags will still be created.
If you don't set this URL, this plugin will point the og:image and twitter:image meta tags to an image in the content of your post or page, if one exists. If it does not, these meta tags will not be created.
This plugin has been developed for a single blog site, not a multisite setup.
If you have already posted a link to Facebook, Facebook has stored that meta information, and will continue to display links to it the same way. Refresh this information on Facebook, at https://developers.facebook.com/tools/debug/og/object/
The description that will be shown when crossposting is limited to 200 characters, but may be shortened to cut off after a word, rather than in the middle of a word.
Before cards will be created for links to your pages, you will have to request approval for your website from Twitter, at https://cards-dev.twitter.com/validator