I've never been happy with the use of a single word to describe associations on a social network, whether that word is "friending" or "following." Human relationships have a huge range of possibilities, and we use subtle variations in wording and adjectives to add nuance to our descriptions of them. To take just one example, "godmother," "stepmother," and "mother" all convey a beloved relationship, yet with vastly different levels of parental (and biological) involvement.
On Twitter I've tried to use Lists to broadly categorize those I follow into groups, mostly focussed on topics. An account can have at most twenty lists, so I've tended to be sparing in creating them. As Twitter has recently de-emphasized lists in the UI, I don't anticipate much more development there.
In the past week I've spent a lot of time on the Google+ Field Trial. There are a number of things I like about it, but one favorite is that I get to name the Circles I use. I can choose the terminology to define our relationship, how I see it from my own perspective.
I also use circles to focus on particular topics, or on groups I am associated with. I rarely post to these circles, mostly just read.
Its very liberating to be able to put a name on one's associations.
footnote: this blog contains articles on a range of topics. If you want more posts like this, I suggest the Social label.