Twitter, that is.
Great service, and capitalized to the tune of something like $20 [update: Er, million] last time I heard. But they still apparently don't have a revenue plan, and the ideas being thrown around (premium paid services for corporate customers, etc.) don't make that much sense.
Here's what DOES make sense:
A lot, maybe even most tweets include links. People see something interesting, they tweet it to their followers with link. People blog something, they tweet it to their followers with link. People arranging a meetup, they tweet their followers with a link to a map. Looking at my Twitterfox feed just now, every other tweet or so has a link.
Some of the links are pre-shortened by the users using TinyURL or Snipr or whatever. The long ones are automatically truncated by Twitter itself.
So, since their design already allows them to modify links, why not modify EVERY link Adjix-style, so that those clicking thru on it see an ad in a small strip at the top of the site they're going to? Then sell the advertising space on a CPM or PPC basis, of course. They could probably even make the advertising contextual based on the link destination or on information they've teased out of the tweeting and clicking users' relationships and behaviors.
For that matter, they could also override into the individual's tweetstream themselves -- every 5th tweet you send, every tenth you receive, you receive an "ad by tweet."
Doesn't seem like rocket science.