Tumblr is a photo/quote/link/chat/audio/video microblogging site. It's made for public, quick, bite-sized browsing, and so far, many of the people who share there seem to post beautiful, evocative, or well-crafted content. It's easy to share content from the iPhone, Android, and an internet browser, and it's easy to customize. The interface is clean, and almost fun. The site's dialogue buttons are fresh and informal but clear and welcoming. Let me reiterate, though, it's a public space. You can choose NOT to let search engines index your blog, but anyone who wants to can find your content.
|screenshot from Tumblr popular feed|
Posterous, on the other hand, seems like more of a productivity site and less like a microblogging site. At least, that's what it seems to brand itself as. You can choose who sees any given blog, or space, as its called, and you can upload anything you want to the space, at any time. You can even schedule content to appear in your space at a certain time of day. It can be public, if you wish, but the option to make your content totally private indicates a different purpose than that of Tumblr. It's a group sharing site, where that group is usually delineated by the user, instead of by the site. Posterous enables public sharing, just in case you want to go that route.
|screenshot from Posterous popular feed|
So: if you need a site for sharing content with a select group of people, go with Posterous! If you like the idea of the Twitter microblog site but appreciate more visuals, or at least more multimedia content, check out Tumblr.