I guess it is more following of people of certain topics that talk about them in twitter. Like someone follows few people abour their hobby, some politicians maybe, and people of other interests they have. I haven't really gotten the idea that people use it to talk to friends per say, or follow them, but for sure people do that as well but maybe it is not the main point.
Though I could be spewing shit because I don't use it. Just the vibe I've been getting over the years.