The number I use for modeling is 300 million for the global general web audience that matters. This is probably considered the active user number but this is the number that's important because this is where you'll generate revenue. For the US market it's 150 million users, remember that we just passed 300 million in total population so we're saying that roughly half of the US is online (which sounds about right).
I think the estimate for total web users world wide is 1 billion, a milestone that was reached last year. Demographics are broken down by country/language but also the "home" and "work" audience. Clickz has numbers if you want to go deeper.
Sorry for yet another random thought but I've been thinking about the idea of "user"/"customer". Everyone has their own definition of user/customer. If you have a log-in, is that a user/customer if they signed-in and never come to your site again? Are you a user of a search engine if you visit that search engine once a year or do you have to use it several times a week before being considered a user? The reason I bring it up is, you almost have to be a user/customer of every service out there, at least all the big companies because they're all in their own little networks.
This means that I'm a customer of Microsoft, Yahoo!, Google, AOL, Netscape, WordPress, SixApart, your blog site, Amazon, eBay, YouTube, MySpace, Friendster, etc, etc, etc. Supposedly the magic number is 2. If you have 2 services within a network you're locked in and truly considered a user, a valued one anyway.