Pageviews/developer isn't a metric that translates very well - there are too many parameters that it doesn't capture well, complexity for one - the Reddit example is a good one. It might be a better argument for increasing the sysadmin headcount, but won't help in your arguments for more development staff.
A well-built website should be able to run itself with only minor interventions as needed. If the company isn't making plans for major new development, then management may well view current headcount as being appropriate.
What are your company's goals with respects to new development in 2011? Are they planning on maintaining the site as-is with minor tweaks? Or are they planning major revisions? Is there a new strategy/initiative in the works?
If they're tasking you with far more work than & the team are capable of, then you have an argument you can make. Otherwise, if you're itching to be a part of some major development initiative, you should probably polish up the resume & look elsewhere.
@Anthony, I've read your responses to my post, and to a couple of other posts below. I've been in your position.
What you're dealing with is a codebase that has evolved to become a Big Ball of Mud.
Unfortunately for you, management/stakeholders have decided to run with it as far as they can go. That's fine, that's their business, they've made the decision to do so with the best possible information at their disposal.
The job of technology (you? is there a CTO or a more senior person above you representing tech?) is to make management understand that the current codebase is hitting the limitations of its' design, and can only go so far. That stability has become compromised, and that tech is simply in maintenance mode trying to keep things running.
In that process, solutions need to be proposed, with pros & cons, required resources, cost estimates, etc. You're trying to persuade them to take an entirely different approach, one that is risky, expensive, and takes away resources from other projects.
This isn't easy, and requires a tremendous amount of support from senior people to make happen - without buy-in, it won't happen. What you're talking about is replacing the core of their web business, a core that was extremely expensive in the first place. That makes people extremely nervous. Think about your mechanic telling you that your beloved 12-year-old car needs a new engine.
If you decide to go this route, good luck. It's not for the faint of heart, but if you can succeed, it's a major accomplishment.
With that, I think it's extremely important from both a career and a sanity perspective to realize that some situations are simply not fixable at your pay-grade, and if you cannot effect change to make those situations fixable, it's time to look elsewhere. Life is too short.