Tumblr is a kind of weird setup, it’s true. Its system really isn’t optimized to support community. For example, it’s extremely difficult to carry on a sustained, coherent conversation with another individual, as you can in a comment thread on a regular blog or Livejournal.
Tumblr is, by design, an innately public forum. Posts are set up so that the original poster is essentially standing on a podium talking to a roomful of people, and then the entire crowd shouts back at once. As opposed to a classic blog structure, where comment threading leads to a situation where effectively you are giving each individual personal time in turn (though others can circulate and come and go in those conversations if they wish).
So Tumblr curtails the ability of people to exist to one another as individuals, rather than faces in a crowd. It definitely isn’t ideal.
Although social media as a whole has a built-in issue where, if you don’t speak up or interact, you effectively don’t exist. In real life, even if you’re not talking, you exist as a physical body that can be seen and takes up space. On the internet, there is no physical presence. People can only know you through your activity.