How else would you do it?
When I stated that the middle is roughly exponential, this was the graph that I was looking at:
d <- density(karma)
plot(log(d$y) ~ d$x)
I don't do this for a living, so I am not sure at all, but if I really really had to make this formal, I would probably use maximum likelihood to fit an exponential distribution on the relevant interval, and then Kolmogorov-Smirnoff. It's what shminux said, except there is probably no closed formula because the cutoffs complicate the thing. And at least one of the cutoffs is really necessary, because below 3 it is obviously not exponential.
I finished creating the 2012 edition of the Best of Rationality Quotes collection. (Here is last year's.)
Best of Rationality Quotes 2012 (500kB page, 434 quotes)
and Best of Rationality Quotes 2009-2012 (1200kB page, 1140 quotes)
The page was built by a short script (source code here) from all the LW Rationality Quotes threads so far. (We had such a thread each month since April 2009.) The script collects all comments with karma score 10 or more, and sorts them by score. Replies are not collected, only top-level comments.
As is now usual, I provide various statistics and top-lists based on the data. (Source code for these is also at the above link, see the README.) I added these as comments to the post: