Tumblr includes a number of social tools and features enabling you to link blogs together and interact with other users, but there's no specific "community" feature. Communities on Tumblr are typically based around a blog, a topic or a tag, and there are many existing examples to explore.
One Blog, Multiple Members
You can add any number of secondary blogs alongside your primary one (the Tumblr you created when you first registered on the site), and these secondary blogs can be configured as group blogs. This means you can invite other users to be admins or members of the blog with posting rights. The Ask and Submit options (accessible via each blog's settings page) can be configured on any Tumblr site to enable users to interact with the blog and submit posts, even if they aren't admins or members.
Tumblr blogs can be considered part of a network covering a particular topic, and you can view some examples of topics or themes by visiting the Spotlight page from the "Find blogs" section of the Tumblr dashboard. Books, food and drink, sports and nature are examples of the categories that are available, and in each case you can view a selection of blogs hand-picked by Tumblr staff.
Tags are another way in which Tumblr blogs can be linked together to form communities. Tags can cover everything from broad themes like "tech" to individual TV shows, artists, celebrities or events. Run a tag search from the Tumblr dashboard to view all of the recent matching posts (and relevant blogs) under that tag. The official Tumblr Explore page shows a rotating mosaic of posts matching tags that are being used extensively across the network. As with the Spotlight page, there are a number of categories you can click through on.
Other Community Features
Various other Tumblr features enable you to build community in whichever way you prefer. Many of the more advanced Tumblr themes allow you to install third-party commenting systems such as Disqus or Facebook, enabling a discussion to be built up around a particular blog. Many themes also allow you to display a list of followers as a widget or in the footer section, which means Tumblr users can find other like-minded individuals and related blogs. Reading through the notes on a post (its likes and reblogs) is another way of identifying Tumblr users with similar tastes and interests.
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