Liz Gannes reports:

As of last September, Posterous told us it had 15 million monthly unique visitors, with three million of its users accessing its products solely through email.

So Twitter is not buying Posterous for users, since Twitter has 10x the user base.

It makes sense that they're buying Posterous for the design talent and engineering talent of the employees of Posterous. But that's not the whole story.

I think the main appeal for Twitter is being able to test its Interest Graph Advertising Offerings Network on a significant property that is not Twitter:

[Twitter CEO Dick Costolo says] there’s so much promise in targeting users based on an “interest graph -- the set of characteristics that can be gleaned from users’ followers and what they click on. He gets animated talking about a treasure trove of data that’s both revealing and noninvasive.

“These accounts I follow paint a very compelling picture of the kind of person I am, even if it doesn’t paint a picture of exactly which uniquely identifiable individual in the world I am,” he says. “I think that allows us to deliver powerful value to advertisers, and powerful value to those who want to speak freely.”

Twitter and Posterous are both public services that allow interest-based self-expression of writers and many more quietly lurking readers.

In other words, the Twitter and Posterous products provide a spectrum of use cases -- from 140 characters to 140 sentences -- with which to test interest-based, noninvasive advertising on mobile, web, and email.

Once an Interest Graph-based ad network is useful to both of these properties, Twitter has at least two monetization growth strategies:

• Roll out the ad network for third party content providers

• Buy other complementary properties.

So, brilliant move, Twitter. Twitter gets the Interest Graph better than anyone.

Isn't the whole value of an advertising network that you don't NEED to buy a slow-ass blogware via email site to test it out?

You should always test on some third party application before rolling out to all third parties.

A slow blogware-via-email site makes an excellent test case.

So it is a smart move for twitter, but will it be good or bad news for Posterous users?

That remains to be seen.

Twitter claims they will give Posterous users advance warning if they change things.

But we'll see what they actually do when the time comes.

Adam, what do you recommend Posterous users do in the meantime?

Update, July 2013.

Well, Twitter didn't care about Posterous. It shut it down.

Some Posterous users migrated to WordPress; others went to PostHaven; many just shut down.

It's a shame that Twitter just plain shut it down. I really liked my Posterous.

I guess Twitter gets enough Interest Graph signal from follows and link clicks:

http://pandawhale.com/post/23569/scalable-twitter-architecture-to-deal-with-150m-active-users-300k-qps-a-22-mbs-firehose-and-send-tweets-in-under-5-seconds