With publishing power ebbing from the few to the many and AJAX killing the postback there are a couple of problems emerging.

Media outlets who make a living by selling eyeballs to advertisers are having to prove the value of their ad space amid growing competition from their readers!
When pages don't refresh (because of AJAX), the number of pageviews a site gets no longer matters. When something no longer works, people are forced to invent something new. When people invent something new they are forced to actually look at the problem. What have they discovered? There is a lot more measure than just 'how many eyeballs are there'. Things like 'how wealthy or influential are the eyeballs', 'how much do the eyeballs trust the publisher', 'how reactive and proactive are the eyeballs in relation to the author' and most importantly 'why do we keep ignoring the person and focusing on their eyeballs'.
Advertisers are finding it hard to work out who to give their money to. Is google really the best broker of my advertising dollars? Which ad network or publisher can promote our brand and product better?

As a result, commentators are abuzz about new definitions and algorithms to measure all this stuff.

Comscore is apparently working on a 'Web 2.0 Metric'.

".....While page views will not altogether cease to be a relevant measure of a site's value, it's clear that there is an increasing need to consider page views alongside newer, more relevant measures. comScore is proud to continue carrying the torch as an industry innovator with the development of a new suite of metrics that will effectively address the Web 2.0 landscape by including enhanced measures of user engagement and advertising exposure. We will be introducing these new metrics to the industry in 2007."

And Jeff Jarvis from BuzzMachine talks about the Distributed Media Economy

So pageviews are obsolete already, thanks to Ajax and other unpage technologies and to the widgetization of content, functionality, and branding: Again, what’s a ‘page’? Audience measurements are obsolete, at last, thanks to the fact that the
former consumer is now also the creator and distributor: What’s an ‘audience’? Mass measurements are dead, thank God, because we are now joyfully fragmented into the mass of niches: Who’s a ‘user’?

Dion Hinchcliffe posts:

"it seems clear that users, businesses, and other organizations that deeply embrace the fundamental nature of the Web as a communications-oriented platform without any single owner except all of us, will be the only ones able to fully exploit the possibilities for online applications."

I find this last quote interesting. "Without any single owner". But I think it needs to be taken further. Media outlets and bloggers alike don't own their audience. In fact they don't even own their participants.

While people are happy to get trapped in walled gardens like MySpace for now, they will soon realize that blogs are the real social network. While they are happy to subscribe to 10, 100, 1000 blogs now, they will start to realize that there is far too much content and they actually need to subscribe to ideas/concepts/interests - not authors.

So perhaps if the audience is not owned by any single site/source then the metric should not be bound to them either. Perhaps the best way to measure engagement is not by domain, but by concept.