As I have mentioned in the past, I see tags as a means of discovery. When you meet someone, you often meet them around a topic. Whether it be sports, arts, music etc. You may meet someone at a concert and become friends because you both like that music. Later, you'll notice how many other things you have in common and the friendship will expand. I don't think Hubski is any different, people can follow tags in order to find people with similar interests. Then they can tailor their feed accordingly and get rid of tags or people as necessary.

Most people I maintain IRL friendships with, I've met through friends, not through public events. Most people I follow on hubski, I've discovered either because the people I've followed were following them, or, the people following me have shared my stuff with them, and they liked it enough to share it again, thus bringing them to my attention. Perhaps it's just my personal experience, but I think people overestimate the incidence of social interaction forming around topics rather than circulating through individuals.