Facebook must be feeling very pleased with itself. Especially when it looks at MySpace.

In June 2006, MySpace was the most popular social networking site in the USA. Less than two years later, Facebook had overtaken MySpace in popularity, based on monthly unique visitors.

MySpace is still out there. But in June of this year it laid off 30 per cent of its workforce. Thus passes the glory of the world, you might say (especially if you think in Latin).

If there is a hint of smugness on the faces of Facebook's mighty, it might be sensible to temper it with another hint of caution. Because the short history of the Internet is littered with great ideas that came to a sudden stop.

Take the early search engines, for example. eXcite, Hotbot and the like. They were the first, but none of them will be the last; because Google found the right formula and they suddenly looked like dinosaurs confronted by faster, smarter mammals. So, is Facebook a Google or another eXcite? It seems incredible that Facebook would ever not be around (although no doubt people thought that about the Roman Empire once). It's popular with a wide spread of people, of all ages.

But it does face concerns: about privacy, for instance. Will Facebook strike the right balance on privacy, or will they get it wrong?

More to the point, the very novelty of some Internet inhabitants can work against them. Once they stop being a novelty, people don't want to play with them any more – particularly if a newer novelty comes along.

Now, we're not saying that Facebook is finished. Far from it. At the moment, it looks to be in rude health. But it is worth remembering one lesson: Internet creatures have to reinvent themselves for every age, or their days are likely to be numbered.