Terms of the deal are not yet public.
Posterous is a blogging platform with a focus on simplicity. Like competitor Tumblr, it's designed so that users can quickly create short posts. Items can be posted from the web, the Posterous mobile app and from email.
The announcement posts say that the Posterous service Spaces will stay up and running "without disruption", but that users who wish to move off the system will get instructions for doing that shortly. The posts also say that the team is hiring.
Founder and CEO Sachin Agarwal said on his personal Posterous Space, "This is one of the greatest days of my entire life." He says, "There is no better fit for Posterous than Twitter," and that "The people at Twitter ... share our vision for making sharing simpler."
Here's the announcement from the Twitter site:
Today, we are welcoming a very talented group from Posterous to Twitter. This team has built an innovative product that makes sharing across the web and mobile devices simple — a goal we share. Posterous engineers, product managers and others will join our teams working on several key initiatives that will make Twitter even better.
Posterous Spaces will remain up and running without disruption. We'll give users ample notice if we make any changes to the service. For users who would like to back-up their content or move to another service, we'll share clear instructions for doing so in the coming weeks.
We're always looking for talented people who have the passion and personality to join Twitter. Acquisitions have given us people and technology that have enabled us to more quickly build a better Twitter for you.
Posterous reports 15 million unique users.
The company launched out of Y Combinator in 2008. It went through a major conceptual redesign in September 2011, with the launch of the Spaces product. Spaces layered a social architecture onto the platform, mimicking in some ways the Twitter concept of "subscribing" to other users' feeds. In addition to letting users read any other public content, the Spaces product was designed to give users a sense of community and reciprocal sharing.
Posterous still wasn't making money back in September, but the company was doing well, according to Agarwal. He said that paid services were forthcoming then, though they haven't yet appeared. It's a safe bet that any such plans will be put on hold or cancelled as Twitter absorbs the company and assigns its employees to integration efforts and non-Posterous activities inside Twitter.