Posterous is a popular social media tool that has some powerful, easy-to-use features and competes directly with sites like Wordpress and Tumblr. The company was purchased by Twitter early last year, but not much outwardly changed. They promised to give "ample notice" for any changes to the service.
Change is now coming. The company announced, via their corporate blog, that the service will finally be shuttered on April 30th. The website, including all Posterous Spaces, as well as all mobile apps, will officially be shut down so that the company can focus entirely on Twitter. The posting doesn't explain what that means, or what exactly we can look forward to seeing in the future.
Back in December, Posterous created a backup tool that allows users to grab all of their content in a zip file. Those instructions were posted once again in the shutdown notice. They recommend that users migrate their blogs to a site like Wordpress or Squarespace.
Source: Posterous blog