Twitter is growing up by growing a business model. This growth towards creating a revenue stream for the company and its investors can be read fully on its blog.
Twitter will not be charging users, oh no, not at all! It will be using a form of advertising through a system called 'Promoted Tweet'.
Essentially, advertisers will be able to pay for tweets. These 'Promoted Tweets' are ordinary Tweets that businesses and organisations can pay Twitter for the opportunity to highlight to a wider group of users on the Twittersphere (I am now getting all twittered out!).
Twitter HQ will measure whether the Tweets resonate (whatever that actually means!) with users and stop showing Promoted Tweets that don’t. They will be clearly labeled as “promoted” and will be sent to those who follow a relevant brand. Promoted Tweets will also retain all the functionality of a regular Tweet.
Twitter has more in the pipeline as well. After assessing this first stage they plan to allow Promoted Tweets to be shown by third party platforms
If you are interested you can find out more on their blogpost. It certainly seems that they have been thinking long and hard about how to maintain a user led resource that is free at the point of use, whilst looking at the long term sustainability of the company. This can only be a good thing, and begin to ensure that people see Twitter as less of a fad and more of a serious platform for engagement.