How to Add Tumblr Sharing to Your iPhone/iPad App Using ShareKit
ShareKit is an open source framework that can be dropped into any iPhone or iPad app to instantly add full sharing capabilities.
You can use ShareKit to add sharing URLs, images, and text with Tumblr in just a few lines of code.
Step 1: Install ShareKit
To start you'll need to add ShareKit to your project. You can download and install it here.
Step 2: That's it
There is no step 2, after installing ShareKit, sharing with Tumblr will already be included in your ShareKit setup along with Delicious, Facebook, Email, Google Reader, Pinboard, Instapaper, Read It Later, and Twitter.
If you'd like to share directly to Tumblr, see Sharing with a Specific Service in the documentation.