Big brands also pay out a lot of money for pay per click programs, radio and T.V. advertising. Many S.E.O. specialists are believe those who pay for Google Adsense, and get into Google's Radio ( they now have ads with Pandora, Slackers and a few other sites) and their T.V. advertising might be getting pushed to the top as well.
Now how does all this tie into the second part of my title... Twitter is dying for answer to monetize it. Advertising such as facebook and myspace wouldn't work that well because Twitter apps have already been created. These apps simply cut out the advertising. Unless something changes probably the easiest route to monetize Twitter, would be to take the route of many directories, have people pay for higher search spots. Many companies would love for an easy way to get success with Twitter, getting your company or just an individual right on page 1 of a Twitter search would make some companies shell out big bucks. I'd say within the next 6 months to the outside of a year Twitter is going to have to come out with a plan to make some money, so we will just have to see what happens.
Sources: ArcherSEM Article on March's 09 Google Algorithm Change
and Current.com's Report on the 6 year old, his PSP x-mas gift and walmart