Have you ever signed up for one of those dating agencies? No, I haven't either, as it happens, but we know how they work. You fill out a form saying what your interests are, and what drives you nuts, and the agency tries to match you up with someone with similar predilections.

Social bookmarking can work in a similar sort of way -- but I'm getting ahead of myself. Let's start at the beginning.

You're probably aware that when you come across a website you like, and wish to return to, you don't have to write down its address. All you have to do is bookmark it, usually by pressing Ctrl D on a PC and Cmd D on a Mac: job done.

However, this approach has some limitations. Firstly, the bookmarks reside on one computer only. If that's not the one you're using when you want to return to the site, that's pretty inconvenient to say the least.

Secondly, if your computer gets stolen or trashed, your bookmarks are lost (unless you've had the foresight to back them up; I doubt many people do).

Those reasons are good enough in themselves for wanting to do things differently, say by saving your bookmarks online somehow. But there is also a third reason…

When you come across a site you like well enough to bookmark for future reference, that's great. But there's only one of you, and only so many hours in a day. Moreover, because you think the way you do, you're going to search for, or come across, or take notice of, particular websites but not others - meaning that you will probably miss some which could be just what you need. You've heard the saying, two heads are better than one. Well, social bookmarking is a good illustration of that principle. Here's how it works.

Let's say I come across a website I think is wonderful. Instead of (or as well as) bookmarking it on my own computer, I could use a social bookmarking website like StumbleUpon, Diigo or Delicious to save it there.

In order to help me find it again, and to help other people find it, I can put tags in the description box for the bookmark. (If you're not sure what tags are, see the article on tagging in this series.)

Example of a social bookmarkOnce I've bookmarked the site I've discovered, I can let other people know about it, in various ways. For example, people can subscribe to my bookmarks (and I theirs), and I can set up Delicious and Diigo to alert people in my Twitter network automatically. I could also, if I wanted to, embed my latest bookmark updates to my website through the use of the update's RSS feed. You can see why it's called social bookmarking.

But I can go even further, and here's where the dating analogy comes in. One thing I can do is click on the tag I've used to see what other people have found on the web and tagged using the same descriptor. And let's say I realise that one person in particular seems to consistently bookmark websites I will find useful. What I can do is hook up with that person by subscribing to their update feed, a possibility which I've already alluded to. OK, it's not as potentially romantic as dating, but I think you'll agree that the analogy works!

Another nice illustration of, if you will, the corollary of following someone's bookmarking activity is to be found in this advertisement for British Telecom, made during the 1980s. If it were made today, and if it concerned websites rather than household appliances, Mrs Jones would be a person to subscribe to!