Focused on group sharing since late 2010, three-year-old social blogging platform Posterous has now launched Posterous Spaces, a group-sharing platform across web and mobile devices that simplifies private sharing with different groups in your life. Also launching today is a new iPhone app that gives mobile users the same sharing abilities as the website (new Android app in the works).

Those with multiple "spaces" can manage them on a dashboard and auto-post updates to over 26 social services. Users can call follow different spaces and create a permanent archive of posts so they don't get lost in a stream. To help the less tech-savvy, Posterous optimizes content before embedding it in a space.

Google+ has made private and segmented sharing easier already, and Facebook recently launched more straightforward private sharing options [ see our story ].

But as Posterous founder and CEO Sachin Agarwal tells LAUNCH, "It's unclear if normal people have figured out Google circles."

Indeed, Sachin is focused on normal folks, not techies. "If I share with my mom, it's really important that it's obvious to her what's going on, what the privacy [level] is and things like that. We really built something that maps to real life and ways that people are used to sharing," he says.

As Sachin points out, Posterous remains open to those who have not registered with the service, which means friends and family can receive and share photos with a Posterous group by email -- still the most popular way to share privately.

Driving the focus on private sharing: 74% of Posterous's growth is from group sharing and private group sharing is growing three times faster than public group sharing.

The most popular type of sharing on Posterous is family vacations and events, followed by photo sharing among classmates and ex-classmates and interest groups like book clubs.

Posterous has over 15M users per month, 12.5M on the web and 3M on email.

Public sharing is still part of Posterous's offering, however. Sachin says those users "have been transferred over seamlessly" and have not lost any functionality. Rather, he says their experience has improved because of the new social features.

Sachin notes the company focused on the iPhone app first because it already had great iPhone developers and "we saw it as a platform where we could really highlight rich media and beautiful design."

[ Screen shot courtesy of Chris Overcash. ]

Posterous went through Y Combinator in 2008 and has raised $5.1M from Redpoint Ventures, Trinity Ventures and angels including Mitch Kapor, Guy Kawasaki and Timothy Ferriss.

Sachin said he has "nothing to announce" about Posterous raising more money.

For more coverage of Posterous' 'Spaces' release check out Tomio Geron's article on Forbes.

LAUNCH Blog