According to The Beat's analysis of DC Comics monthly sales here, the best selling comic of December 2006 was Justice League of America #4 at over 130,000 copies. The top comic of the year was Justice League of America #1 which broke 200,000 copies.
The typical DC superhero book seems to be orbiting in the range of 20,000 to 40,000 in sales. In the early Seventies -- when I was a child and therefore everything about life was perfect -- Jack Kirby's "Fourth World" books (New Gods, Mister Miracle et al) sold in the neighborhood of 200,000 to 300,000 copies and were cancelled as failures.
It would have been typical for an issue of Superman to sell over 400,000 copies during the early Seventies. That would be an average regular issue, mind you, not a "first issue" or "special event" issue. More historical perspective on patterns in comics sales can be found here.
(Why were Kirby's books considered failures? This is a source of great contention among Kirby fans -- ah, but what isn't? -- but the general impression is that it was because his books were selling less than 50% of their print run. DC could simply have lowered the print run to match demand...but the real problem was that their expectations had been unrealistically high in the first place. If you're expecting the absolute biggest success in comics history and get merely a respectable success, it seems like a failure. But I digress.)
You may rightly point out all the other ways today isn't like 1971 -- comics cost more, they aren't sold at newsstands, I'm old enough to shave, and there's now such a thing as "waiting for the trade paperback" instead of buying mothly issues. Be that as it may: we still have an idea that the current "universe" of mainstream monthly comic book readers is going to be something considerably above 100,000 readers.
DC has a number of books that start above the average in sales and then lose one or two thousand readers per month. A lot of them seem to have had a surge from the "jumping on board" aspect of the "One Year Later" scheme...but failed to hold those readers, and didn't continue bringing in new readers to replace those defections.
Here's a really important clue for comics publishers: "Jumping on" issues don't work, and if you have to have one in the first place, you're already doing something wrong. People do not mind coming into the middle of a story, if it's a good story and being told well. You catch the tenth or twelfth episode of a TV series and say "Wow, this is really good, I gotta start watching this"? Or flip the channels and come across the middle of a movie that hooks you, and you realize you want to watch it from the beginning? These things happen all the time. I've opened up novels at random in the middle and gotten an impression of the writing that made me want to get the book. Not knowing who the characters are or what the situation may be offers no impediment to the newcomer provided you see some spark of entertainment that makes you want more. All your "jumping on" issue does is service the new reader who already knows this is a "jumping on" issue -- which assumes a higher level of familiarity with that title among people who aren't already reading it than one ought to assume -- or the few who happen upon it that particular month by chance. You expect every potential reader is that person? What manner of god are you, comics publisher, that you may control destiny thus?
Several of the lowest sellers are Cartoon Network tie-ins. Krypto the Superdog is selling under 10 thousand copies. Scooby Doo is under 5 thousand. Looney Tunes under 3 thousand. CN Block Party less than 2 thousand. These numbers are appalling. If I were a higher-up executive at Warner with no interest in comics, looking at DC's performance with these books that are essentially getting constant free advertising on television, I'd have to conclude DC was incapable of selling bowls on a day when it was raining soup.
Kids who watch the cartoons are not going into the comic shops and/or not finding those comics. I'm not saying the comics aren't there; I'm saying the kids aren't finding them. And you know what? If I were a parent, I wouldn't let my kid go into a comic shop alone. Assuming there was even one in our town, which (statistically speaking) there probably wouldn't be.
But every time I go to one of the supermarkets in my neighborhood or to the local K-Mart, my attention is always fixed by the rack of Disney and Archie comics digests in the impulse purchase racks at the checkout lines. They look appealing, they look affordable, and they look parent-friendly. Attention DC, another clue approaching: put your Cartoon Network tie-in titles into digests and do whatever it takes to get them onto those racks! I bet you'd sell more than 2000 freaking copies.
(Update: Kevin gently points out something I have overlooked about this in the comments section.)
There are two issues raised in the above: one is how small the total universe of current comic book readers has become relative to its size in the past, and how the mainstream publishers have failed to enlarge it...and the other is how well or poorly that shrinking universe has been served by the publishers. Price and availability are the keys to the first issue. Blaming video games as competition for reader dollars doesn't really cut it. There are always things competing for your prospective customers' wallets, and there always will be. Suck it up and accept that; don't use it as an excuse for failing to get your product in front of people. And if there's so much pricey competition, how does continually raising the price help? As to the second issue...
It's not an artistic or aesthetic value judgement to say that Marvel and DC are simply not giving the existing comics reading audience what they want. It's a statement of objective fact, demonstrated empirically by sales figures. This is axiomatic when you see two thousand more people each month saying "you're not giving me what I want in this comic" and an equal number aren't replacing them. Finding out what it is they want and giving it to them is the job of publishers and editors and their creative personnel. And it's a job the majority of them just aren't doing.