Twitter has always been the social networking site to own the @mention, and now that a bunch of other social networks (including Facebook) have integrated it, Tumblr has become the latest site to make @mentions work its own platform.
Anytime you want to mention a specific Tumblr user in a post, simply type the "@" symbol and the first letters of the username. A dropdown list of usernames will automatically appear as you being typing the username you want to mention.
When when you make a new post with an @mention in it, the mentioned user will receive a notification in their dashboard stream and activity page.
Unfortunately, for now, use of @mentions are limited to only the web version of Tumblr, but considering how heavily Tumblr relies on activity through its mobile apps, we can only assume that the @mention feature will be rolled out on mobile sometime soon as well.
Photo © Tumblr