One of the fun new features is the member list, which, as the name suggests, displays the name and number of posts of every registered TWoP user. It's thanks to this nifty feature that I was able to determine that I currently rank 124th in total number of posts on TWoP, with 2,388 posts. The number one poster, g-girl, has 13,424 posts, more than five times as I. Among people I've met in person, the highest ranking poster is brooding_soul, who has 7,909 posts and ranks 6th overall.
Further screwing around indicates that there are 29,228 registered TWoP users, which puts me in the 92nd percentile of all TWoPers. I also determined that I was the 258th person to register as a TWoP member. Not bad.
ETA: caltrask55's comment suggests that it may not be immediately obvious how to figure out your rank. First, find out how many posts you've made. The easiest way to do that is to find one of your old posts and click on your name to see your profile. (If necessary, make a post in the Test forum in the Nuts and Bolts area.) Once you know how many posts you've made, click on the Members link, which will display a list of the first ten posters in alphabetical order. Go down to the bottom of that list, change "Member Name" to "Total Posts" and "Ascending" to "Descending," and click Go. Then browse through the list until you find your name. You can jump forward by larger amounts by clicking the word Pages and entering the number of pages forward you want to jump.) Once you've found the page on which your user name appears, it's a matter of simple arithmetic to determine your overall ranking. In my case, I was the fourth person listed on the thirteenth page of ten users, which makes me the 124th most prolific poster.