They probably bought it for the people. So there would be no one left to work on the Posterous site if everyone is working on Twitter instead. I know they didn't buy it for the infrastructure since it's hosted on Rackspace and Twitter is an AWS shop.

https://blog.posterous.com/thanks-from-posterous

But it all seems very volatile as people tend not to stay in one place for too long. Oh well ...

In my opinion, this is a staff acquisition. Buy a company to get the staff free and on board, ditch the product/project.

Besides, I think Posterous would be a good complement to 140 char-twitter. I don't even consider them as competition to each other..