"Twitter, which is preparing to offer shares to the public in an initial stock offering, has struck its own deals with about 250 cellphone companies in more than 100 countries to offer some free Twitter access, and worked to make sure its service is easy to use on even the cheapest cellphones."

http://www.nytimes.com/2013/08/21/technology/facebook-leads-an-effort-to-lower-barriers-to-internet-access.html?hp