Jamie Pinkham:

ShareKit aims to be a drop in service for pushing text, files, urls and images in iOS applications. I wrote the Tumblr “Sharer” which allows you to share images, text and urls as photo posts, text posts, and link posts.

Some limitations, however:

1. Can only post to main tumblelog.
2. Can only publish immediately (ie, no draft/queue/publish on support).

This framework is a great way to add drop-in social networking to your iOS app!