Update Facebook Open Graph meta tag through JavaScript
I'm guessing you figured this out already, but others may have the same problem.
You can't change the Open Graph meta tags with JavaScript after the pageload, as Facebook requests the page from the server again when looking for the meta tags, and so it will not find the updated tags.
Furthermore, I found that Facebook only searches the header for the Open Graph meta tags, so adding them in the body will not work either.
Facebook scrapes pages for meta-data only when it is necessary. After you make changes to the metadata, you need to resubmit the object with the 'scrape' parameter set to 'true' in your POST request.
This is outlined in the Open Graph documentation for updating objects.
The meta tags are read by Facebook when shared (liked, etc.), so the JavaScript code might not work. You can test using the online debugger.