What has everyone been using to promote their comics? Facebook? Other sites like Drunk Duck?

Project Wonderfull has been pretty good to me, I've spent a fair bit on it and not gotten too far. You have to buy your ads wisely, purchase stuff based on how similar the comic is to yours, what size the ad is, and where on the site the ad is. Using google analytics in conjunction with this helps, you can how many people who come from sites your advertising on will stay so you can work out what ads are most effective.
I keep some social buttons down the bottom of each comic and every now and then someone posts it to facebook or twitter and I get a few hits.
Aside from that I have no idea.

Awesome, thanks for the info. I've heard of Project Wonderful, I'll give it a look.

I will second project wonderful.  very useful.

Yeah Project Wonderful is ok, but I've learned recently that after a while your prmotional efforts (and potential ad revenue) with them sort of "plateau". Promoting in any and all forums you can find is certainly helpful. However, I get most of my big page views from Reddit, granted very few of those hits become sticky readers. But still, if you need a day with a couple thousand extra hits, Reddit is very helpful. 

Generally Facebook, Twitter, and DeviantArt.  Doesn't work too well, but it's better than nothing.