You can also look at Google hits - 752,000 for "freerepublic.com" vs. 246,000 for "democraticunderground.com".
The Free Republic obsession with Democratic Underground has always bugged me. Why do people have the idea that the sites are comparable? FR is at least relatively mainstream, while DU is pretty marginal. Of course people will find something to be mocked of on DU - you can find something to be made fun of in any small community of zealots. If people assume that what you read there is representative of Democrats or liberals in general, however, they are deluding themselves.
For what it's worth, I am a longtime liberal lurker on FR, and I stick around because there are smart people here with well reasoned opinions on a wide range of topics. I refuse to believe that one side has a monopoly on the truth (or more fundamentally, that buying into the liberal/conservative Democrat/Republican dichotomy is a smart approach to begin with.) Though I frequent many political websites, both on the right and left, I had never heard of Democratic Underground before I saw it linked to from Free Republic. DU is neither important nor representative in the world of lefty political sites. If all you're looking for in political discussion is to take potshots at freaks, then by all means have your fun with DU - but please don't take it too seriously.
Then you're in denial.
FRee Republic is all about conservatism. DU is all about liberalism.
Bottomline. Conservatism trumps liberalism ALL DAY LONG!