Normally I'd see somebody coming out and saying 'this is a replacement for X' to be both desperate and a little misguided; desperate because they're putting their (more successful) competitor's name right there in the title, and misguided because by setting yourself up as a X Replacement you quite often both establish yourself in the audience's mind in the exact same space and context as the established service, and also basically communicate: 'You already use X. But you shouldn't. You should stop right now.' Rather than say, 'Look at how cool Y is! You are probably so interested you're going to sign up right now and if you like it, maybe you'll forget about lame old X!'
In Posterous's case, though, they just keep churning out such powerful functionality that the reader ends up agreeing, 'Well hell, why _don't_ I use this?' A look at the official blog just shows you major, polished, easy-to-use feature after MPEtUF.
(But if they don't add Markdown support soon I'm gonna cry.)