Twitter has looked for a reasonable business model and has elected to sell advertising on their site. It is a choice doomed to piss off a lot of users when the time comes.
Twitter has three types of users: personal, business and spammers. The spammers are ritually thrown off as quickly as they sign on. The business users take advantage of twitter’s real-time presence to advertise and offer customer service. The personal users just want to have fun, meet new people, share ideas and generally shoot the breeze.
If twitter wants to make businesses pay for ads, they should simply make them pay for access to the service. A small fee for the millions of businesses that access twitter should generate more than enough income, allowing twitter to stay basically the same for the personal user.
Let the web stores, brick and mortars and internet consultants bear the burden while I and my friends keep the freedom to follow or ignore their streams.