This post is two parts Webcomic Writer, one part Webcomics Guru.
I've had a couple of emails asking me why I haven't added Vote buttons for BuzzComix or Top WebComics to the site. They were on the old site, after all. Why aren't they here? The answer is quite mind-bogglingly simple: Nobody looks at top webcomic listing sites except for other webcomic artists. It's an incredibly incestuous form of marketing that doesn't really draw in any real readers. Not to say that other webcomic peoples aren't real, of course, but when your entire fanbase is made up of people who are busy working on their own webcomic you have to wonder how many people are actually reading your comic.
Let me put it this way: very few television actors actually watch television. Very few writers get a chance to sit down and read a good book. And, with a few notable exceptions, not many web cartoonists have the time in the morning or afternoon to sit and scroll through all of their favourite webcomics. It's just the nature of the beast.
In short, webcomic listing sites are far too nichey to really work as a promotion tool. It's sites like Digg and StumbleUpon that ultimately make the bigger difference - webcomic listing sites simply don't reach out to a varied enough audience.
I've contended on webcomic forums this very thing, and that we need to reach outside our little community to find readers that become fans because they like the comic, not because we're art buddies. I think that's the beginning of building a real fan base.
Nothing wrong with other cartoonists reading you, but we've got to find ways that reach outside our nichey places, as you say. SU has brought me some decent traffic on some days. I'm not sure how many stick, but at least it's the right kind of person - someone looking for something new.