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I imagine this alone would encourage users to be online, and may account for the rise in searches.
What is the most important reasons for it ?
In April 2004 the majority of people in UK who actually had internet were still on dialup.
Soon afterwards the price of dialup and basic broadband became more or less the same.
By April 2008 the majority of people actually had internet, and it was broadband.
In that time many households went from one computer to "laptop for the wife, laptops for the kids".
There was a cultural shift, an explosion rather than a gradual increase.
this number is "searches per searcher" but not "pv per internet people"
If you thought of something to search for in dialup days, you had to initiate a connection, listen to the crazy noise the modem made, wait for a browser to launch on a slow computer, head for your preferred search engine and join the "world wide wait".
These days you just type your search term in a box at the top of the browser for instant results.
Also, people are more used to how it works - whatever the question, they "just Google it".
I would also not assume that USA in its entirety (it is a large and diverse country) is necessarily more internet-developed overall than UK (a very small country with a large percentage of the population concentrated in a few cities).
One factor that may have been relevant is the US habit of using "search term dot com", otherwise known as "type in traffic". I have never seen this in UK, where Google is a national obsession (allegedly 90% plus market share, which I find easy to believe).
Good luck with your thesis, I'm sure you will get high marks.
User goes to Google.co.uk and searches for a product, they get lots of results, but realise they are searching the web, they then check the "results from the UK" checkbox and click search again.
This is just something I have noticed when I watch other people searching. But I think it may well be a factor.