Tumblr is now receiving a legitimate instant messaging feature and it's arriving eight years after the site launched. The rollout of this feature is moving slowly, meaning only a small portion of Tumblr users can actually send IMs right now. Everyone will have access to the feature in "a few weeks."
In your blog's settings area, you can adjust your IM privacy settings so that you only receive messages from the users you follow.
The Tumblr messaging tool is arriving on iOS, Android, and Web. Once it is available, you will see a "little smiley balloon icon" on your dashboard.