Posting the main url of drupal site on facebook wall
You have a two part question here.

One, the best way to deal with OG info on your site is with the Metatag module:


  The Metatag module allows you to automatically provide structured
  metadata, aka "meta tags", about a website. In the context of search
  engine optimization, when people refer to meta tags they are usually
  referring to the meta description tag and the meta keywords tag that
  may help improve the rankings and display of a site in search engine
  results. In addition, the module provides support for meta tags (Open
  Graph Protocol from Facebook, Twitter Cards from Twitter) that allow
  control of how content appears when shared on social networks.


When you install and enable this, you can customize what OG info appears where, and it has full token support for dynamically creating the meta elements pages on a page's content.  You can also hardcode values for specific cases, like the home page.

The second part of your question has to with the fact that FB heavily caches page contents.  The provide a debug tool for dealing with this:

https://developers.facebook.com/tools/debug/

In addition to seeing exactly what OG info it is pulling in about a URL you provide, it has the side effect of clearing the cache for that particular URL.