The design of Tumblr, the blogging tool and social network, is guided by feeling. In particular, the feelings of David Karp, the company’s 26-year-old founder, whose instincts tend to run counter to current Web conventions. Tumblr does not display “follower” counts, for example, or other numerical markers of popularity that are viewed as crucial social-media features, because Karp finds them “really gross.” The culture of public friend-and-follow reciprocity that theoretically expands a social networking service can, in his view, “really poison a whole community.”