Force the feed to show the FIRST post in a discussion topic

[Good Gooder]

I use feedburner to take the URL of a new topic post to a forum and to make a facebook status containing a link to that topic. Included in the Facebook status is the Topic Title and the Full Post content.

The problem I have is that Feed burner waits 30 minutes before posting the link to Facebook and if someone replies to the forum topic before that time then the most recent forum comment is posted to Facebook instead of the original post.

So, how can I force feedburner to only send the original/1st post in the topic to Facebook?

Thanks very much for any help with this
Thanks
sg