The basic function of these sites is to show people you follow (receive updates from) who aren’t receiving your own updates.

So what’s the point of that, you ask?

Well, like a lot of social networks, on Twitter:

(a) the number and quality of followers you have is an indication of status in this rather insular social media world. (Let’s forget about the recent arrival of the UK’s current Stately Homo Stephen Fry onto the scene).

(b) following someone is an indication of like and respect. I care enough to hear what you’re up to.

So if you follow someone, and they don’t follow you back (you get sent an email to say X is following you), then it appears, sort of, that they don’t like or respect you very much.

You could do this quite easily before by clicking on the ‘followers’ link on your twitter home page, but these new sites make that information a lot easier to take in. A bit like in sites such as Facebook — you can see if someone hasn’t responded to your friendship request. But the thing is that Twitter itself has — historically — never shown any of that information in a way people could really take in and analyse. So it might come as a bit of a shock to some people that prominent twitizens (oh, yes) aren’t hanging on their every word.

People might get upset when they discover they are following people they thought had become friends (and probably are) but that those ‘friends’ are not following them.

However, there are a few other possibilities:

• they are already following 200 other people and the noise from that is enough, thank you very much.
• Dunbar’s Number — more than 150 people is more people than they can maintain stable relationships with. There are already 150 or more people they are paying attention to. It’s a psychological impossibility to take on someone else, no matter who.
• they like you very much, but going in and following you and dropping someone else is too much trouble.
• they auto-delete ‘new follower’ emails. A strong temptation if you get more than one a day.
• they forgot.
• they don’t care nearly as much about Twitter as you.
• they have already linked to you on Linked-In, friended you on Facebook and subscribed to your RSS. That is enough.
• they have a million far more important things to do that affect their ability to carry on working.

Elsewhere: Paul Walsh on the damaging effect of this and Charles Arthur on (slightly disputed) best practice.