Have you noticed how we’re getting more and more INBOXes? Every damn Web service you sign-up builds messaging into it. The messaging system usually comes with some form of notification that lands in your email INBOX. And it most certainly always will make you visit the website to read the message.
Facebook is half-way there. They already set Reply-To correctly. They just refuse to paste the message in the notification. Flickr is also half-way there – they paste the message in the notification email, but to reply, you need to get to their web interface. Though within some of my archives, they’ve set Reply-To correctly (making Flickr, get it; now you deal with the duplicate on your web inbox…). Twitter sends the contents of direct messages via email entirely, and to reply, you’ve got to use the Twitter interface (though in this sense, I guess there’s some usefulness in it).
Its not only messaging that some provide. You might get comments to approve, or someone might “write on your wall”. Or a “shout” (on last.fm). Sites like last.fm for me are so passively used, that having an Inbox there is really silly.
Ideally, I’d deal with everything via email. Full contents of the message, with reply-to set sensibly. But I guess this doesn’t suit the modern crowd’s workflow, who login to Facebook, or Friendster or the social-networking-hype-du-jour on a daily basis?