Maybe administration can find out. Looks as if there are two different 5,000,000th post. Each post carries it's # within the address to the post, for instance this post is #5114051. But their is a 112,000 difference between that number and the number of posts claimed. I don't know, this may be because of deleted posts. Anyhow the actual 5,000,000th post was made on 3-27-2011 around 11:30 am my time(mountain standard).
Here is the post that is just 21 shy of 5 million
http://www.monsterfishkeepers.com/forums/showpost.php?p=4999791&postcount=17