This is a question with no answer, depending on what you mean by "Internet" and "used." But has anyone who publishes numbers on the size of the Internet and also tracks usage ever released a number such as this? If there are 10-20 billion pages, and roughly 1 billion users [1], then each person need only surf 10-20 pages to get to the entire Internet. But there is a very long tail of usage, and popular websites dominate.

I found one stat [2] that showed over 500,000 Americans visited over 28 websites in July of '03. Another site [3] says that the top 10 websites count for 40% of all page views. This leaves the image of a very long tail of abandoned websites that are rarely – if ever – visited.

Google could give us an estimate of this number (in theory ;). Take the size of their index and subtract off the number of unique urls that were clicked on.

[1] http://www.internetworldstats.com/
[2] http://www.imediaconnection.com/content/1533.asp
[3] http://www.readwriteweb.com/archives/long_tail_shrinking.php