Over the course of a few months I about halved my following count on Twitter. The application definitely makes it easier to follow people than to unfollow them, but also this mentality of thinking more about the possibly of interesting content than the damage of content that's uninteresting made getting used to reducing my following count tricky.
It is kind of how when decluttering it's not only more effort to clean than to ignore the issue, but it's also easy to fall into the mindset of "I liked/needed this thing that one time, maybe I'll like/need it again sometime in the future".
Judging from posts like these (and the other comments on this thread), most people have difficulty unfollowing others, which is a shame.