Most of these companies ("web 2.0" or not) won't make enough money on advertising, UNLESS they have a plan to offer intensely deep targeting. Targeting and ROI is the new world of advertising.
The top 10 online-ad sellers (including Google, Yahoo, MSN, etc.) currently control 71% of the online ad market. All the growth in the online ad space is NOT in untargeted "brand" advertising, it's in keyword-based CPC (cost per click) advertising where the long tail of advertisers can get their message out there on a relevant basis and pay as little as $50/month! It works for consumers (who see ads that are relevant to them) AND for advertisers (who reach only specific people and pay only for actual clicks).
If you're a long tail Web site relying on your ability to sell ads based on impressions (not on targeting or actual performance) then you WILL have a tough time selling your ad inventory. The new age of advertising is all about results, and if the advertiser doesn't get the results -- they move on. Take the popular TechCrunch Web site for instance; in the last month they've churned all their advertisers except the company Arrington is a founder of. At some point he will have to reduce his rates and/or demonstrate a tangible ROI.
Relying on Google AdSense won't help much either. As people see AdSense ad units all over the Web, they naturally begin to ignore it -- every ad has a lifetime of effectiveness.
The answer lies in coupling the long tail of Web publishers to the long tail of advertisers, and monetizing the [context] keywords and behaviors, not impressions. To date, this is largely done through Google and Yahoo, but the publishers are stuck -- they're left knowing nothing about their users' interests/behaviors.
Others Online takes a different route. We couple a tangible user value proposition to the act of profiling users (we offer a service that shows you people relevant to your web browsing and other interests, point you to their blog, Web site, etc.) We let users control their behavioral profiles, and give partner Web publishers deep insight on their user base (non PII) as well as double their revenue per user.
Users win because it's a way for them to not only promote their Web site and themselves to others, but they also see people relevant to their Google search for instance.