It does have a friends page equivalent, actually -- you can follow anyone on Tumblr with the "follow" link on any Tumblr page when you're logged in, and when you visit the Tumblr front page you get your "dashboard" which lets you make posts -- and shows you new posts from people you're following. (And also shows you when somebody likes or reblogs one of your posts.)
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It does have a friends page equivalent, actually -- you can follow anyone on Tumblr with the "follow" link on any Tumblr page when you're logged in, and when you visit the Tumblr front page you get your "dashboard" which lets you make posts -- and shows you new posts from people you're following. (And also shows you when somebody likes or reblogs one of your posts.)