If your theme is using the get_comments_number() function the result from Facebook servers is cached as a transient for 15 minutes to avoid the overhead of polling Facebook servers with every page load. Your site’s caching setup might cache the page even longer to avoid regenerating the page HTML with every incoming request.
If your theme uses the comments_number() function the comment count is polled asynchronously from Facebook’s servers.
(@sidaanth)
12 years, 11 months ago
When a User Post a Comment in the Facebook Comment Box, the number is not updated on the Post Header Description as shown below:
http://wordpress.org/extend/plugins/facebook/