I’m not sure what people were expecting here. Posterous suffered the unfortunate fate of having a similar product to tumblr with the same plan, but far less adoption. Having raised a sizeable amount of money ($10M) but not seeing huge growth, or any monetization thus far, they’re likely out of options other than acquisition, or shutting it’s doors. The team now likely has good jobs at Twitter, and will likely be able to make a big impact there. Good for them.
My only criticism on the announcement is that’s it has mixed messages for customers. They go from saying “Posterous Spaces will remain up and running without disruption”, and follow with “We’ll give users ample notice if we make any changes to the service.”, and then “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.” This doesn’t really answer the question “are you shutting down?”. Granted, Posterous customers have enjoying have a free blogging platform for some time, and have never paid a dime, so there is a bit of buyer-beware. But, it doesn’t really set a clear message for what customers should do other than hint at backing up and/or migrating to other platforms.