After four years, Twitter begins running ads on its site Tuesday. The ads appear on the popular microblogging site when users conduct a search, much like the model Google uses. In the future, ads may appear in the stream of posts that visitors see when they use the site. Among the original advertisers are Starbucks, Virgin America, and Best Buy. At most, only one in ten users will actually see the ads Tuesday. Since its inception, observers have wondered how exactly Twitter would profit.