The Core
In one short sentence the business model twitter can be phrased as follows: "Twitter is basically a mass-scale marketing platform, in which every tweeter is a marketeer and every follower a set of eyeballs and a potential re-marketeer."
2006 - A Fresh Start
In 2006 Twitter launched and for several there was criticism because their business model or more specific their revenue model was not clear.
You know, people always want to know what a company is up to: 'how are they going to make money?'. And until there is a question people will stress that point.
The founders always said they had no rush in finding a business model. They just followed Google's approach (build a service that many people like to use, and then figure out how to make money with it).
2010 - Growing up
In 2010 twitter started with was Promoted Tweets. Promoted Tweets are paid tweets that can be used by advertisers to appear at the top of a search result page for a period of time. Also they introduced Commercial Accounts.
2011 - Smart Changes
In 2011, Twitter introduced Promoted Trends. Promoted trends are flat-rate daily adds that generate huge numbers of impressions in a day. And they are very expensive.
2012 - Playing Poker
In 2012, Twitter introduced Twitter Cards. With that Twitter follows Google and Facebook's in encouraging greater advertising plus enabling development by others on the twitter platform.
Stay tuned for new updates, because Twitter's business model is still work in progress!