Twitter will no longer allow third party advertising providers to use the api of the service to insert ads within the streams of user updates automaticly. It is a foundamental step, which for sure will cause some trouble to some services specialized to deliver ads on Twitter this way. But it is not a general prohibition of third party advertising services in the Twittersphere. Within the blog post the company names other opportunities like inserting ads manually or reaching Twitter users by advertising on Twitter apps. The company justifies this step by the aim to keep a unique user experience and to foster the ongoing development of the service.