In my Internet Marketing:Strategy, Implementation and Practice book I ask this question in the chapter on traffic building / digital communications. I hoped readers would think of the relative sizes of different types of sites and their representation through different forms of digital media channels both on the major sites and the "long tail" of smaller sites.
So it was nice to see the BBC recently use a similar analogy in their blockbusting Internet documentary, Virtual Revolution.
They had a great visual produced which I've grabbed from the iPlayer version of the show:
Since the programme Hitwise have published a more detailed map, which I thought would be interesting to compare - it's worth reading Robin Goad's analysis on the Hitwise site.
What are the implications for digital strategy?
The programme used the visuals to paint a "The Internet is evil - there is no choice - our minds our controlled by a few major brands line for impact. One quote was 'the web has one search engine, one social network, one store and one auction site'. I think you'll agree that's a bit of a simplification...particularly if you look at different countries and different demographics within them.
When you look at the Hitwise data, the top 100 sites currently account for 42.5% of all UK Internet visits, so they are dominant, but there are a few other businesses who are successful online - including many SMEs who successfully target niche audiences. Many of the top sites are search-social-media where we are likely to make most of our visits, but when we're looking for products, it's a different story.
So, the implications are that you do need to maximise your opportunity to communicate with audiences on the major planets, but you can also be successful by reaching your audience on the asteroids - they can be big when you're standing on them.
To complete the picture, this is the visual from my book - I think I first introduced it way back in 2005 so no mention of social networks in this diagram - must fix that in the 5th edition!