The service Chi.mp launched a while ago. It's pretty Tumblr-esque in terms of the site set ups not being social-network centric, but I don't see any means to post things to it from other avenues.

It is critical for all content heavy blog-type (even micro) to engage their audience AWAY from their site. The ways to execute this (and that are down right necessary for the site's success) are as follows, in my opinion:

• Bookmarket : Programmatically this is the easiest implementation, just a crapload of JavaScript that posts to the site if the user is currently logged in to an auth'd session. Win. Tumblr makes the best use of this, with Posterous on the heals. Even the Firefox extension Shareaholic is a nice way to engage users while browsing.
• AIR Application : This might not be such a huge deal, especially if the user is already at a desktop to use the app, why not just have another tab open in a browser? But all-in-all AIR apps are good foresight for the next piece...
• Mobile Application : I'm not talking about an iPhone app, but that'd be ok if you're into that sort of thing. What's really needed is someone who wants to tend to the global market (Blackberry, Nokia, Palm, Android AND iPhone alike), yea that means you need to develop something on 5 platforms, but in the end, its worth the reach.
• Mobile Integration : In an acronym, SMS. You have to have it. Just ask Pownce how long they think they'll last without SMS while their competition, Twitter... oh wait. See how easy that case study is?

That's really it. If you want use generated content, it has got to be generated from multiple doors, on multiple fronts, using clean/familiar interfaces on multiple platforms.

Pain in the ass, huh? Well, this is business and you need to maximize profits. Only way to do it is to invest in a solid, quality, viable, long lasting product.