title: Relationship between FB Open Graph object identity, object url and app domain?
body: <p>I'm looking for clarification on how Facebook uniquely identifies objects in the object graph.  Specifically, we have several hundred sites for which there is significant overlap in pages that display hotel information.  We'd like to have 'Like' buttons for hotels, but have the counts aggregate across sites - so that it doesn't matter on which site a person 'likes' a specific hotel, it'll contribute to the same global count.</p>
  
  <p>More importantly, the philosophy behind Open Graph is that is represents associations between real-world objects - which just happen to be represented by a web-page, so it makes sense for page about a particular hotel to reference the same hotel page object in the graph, regardless of which site it was 'liked' from.  For example, for books and products the <a href="http://ogp.me/" rel="nofollow">Open Graph Protocol (OGP)</a> uses ISBN and UPC numbers, while no such universal code exists for other types.</p>
  
  <p>Specifically,</p>
  
  <ol>
  <li>If I set the Open Graph Protocol (OSP) <strong>og:url</strong> meta property of a hotel page to a URL that references a central site - so that it will be the same og:url for all pages about that hotel regardless of which of our many sites the information appears on, will that achieve the uniqueness required? (does FB even interpret that or is it considered opaque?)</li>
  <li>When I use the <a href="https://developers.facebook.com/tools/debug" rel="nofollow">FB Debugger/Lint</a> app to check my page, it complains that the App ID referenced in the <strong>fb:app_id</strong> meta tag isn't allowed on any domain other that the 'base' domain set in the App's settings on Facebook.  Assuming that this implies I need to create a separate app for <strong>every</strong> domain we use(!), is there any implication of that for the hotel object identity? (or will the hotel identity remain independent and 'Like'ing a hotel via a Graph API call with any App ID will result in the correct association with the single hotel object as identified in its og:url?).  Do I even need an App ID on the page? (just admins perhaps?)</li>
  <li>If Facebook returns any of the hotels when people search on the Facebook site, which URL/link will it offer for the result? (the og:url?)</li>
  <li>Lastly, and slightly OT, I'm wondering what happens if the og: metadata for a particular canonical og:url object has different properties on different sites, what Facebook will make of it (- e.g. if the og:site_name was different on each domain where the same hotel was represented by the same og:url)</li>
  </ol>
  
  <p>I'm still trying to get my head around it all, so I appreciate all those Facebook ninjas out there :)   Apologies if I've put too much detail.</p>
  
  <p>Cheers.</p>