Posterous could have been growing but if the users were never going to generate any revenue it was inevitable they would have to shut down or be acquired later on. The sale could have been as much for the team (who wants to work on a product that isn't really going anywhere?) as it was for the investors. Keeping Posterous open could have just been delaying the inevitable.
As an aside, Tumblr is apparently having financial troubles (this seems to be speculation based on supposed leaked investor feelings) and Tumblr is huge compared to what Posterous is (especially when you look at how Tumblr cornered the social side of micro blogging and Posterous hadn't).
Having 10 million users is appealing to investors because there's the chance that they might turn into revenue in the future but for a company like Twitter that already has most of those users using their service they're probably not worth very much at all.
Posterous could have been stuck in the stage before becoming a proper business, it could be a fantastic service that tens of millions of users loved but if there's no way to move those users into revenue generating users their most valuable asset could have been the team.
As an aside, Tumblr is apparently having financial troubles (this seems to be speculation based on supposed leaked investor feelings) and Tumblr is huge compared to what Posterous is (especially when you look at how Tumblr cornered the social side of micro blogging and Posterous hadn't).
Having 10 million users is appealing to investors because there's the chance that they might turn into revenue in the future but for a company like Twitter that already has most of those users using their service they're probably not worth very much at all.
Posterous could have been stuck in the stage before becoming a proper business, it could be a fantastic service that tens of millions of users loved but if there's no way to move those users into revenue generating users their most valuable asset could have been the team.