physics - 10:04 pm on Jan 25, 2010 (gmt 0)

Personally, I'd pin it on the death of bookmarks. In the time that I can scroll down or enter a search term into a bookmark field, I can find it in a search engine.

Were that many people really using bookmarks?

An alternative explanation is that as more and more people come online, the average web user is less and less web savvy and thus more likely to type 'yahoo' into google.com search box in order to get to Yahoo. Even that probably can't explain such a big increase.
Another factor might be that Google et. al. have become so good at indexing sites (while on site searches generally still leave much to be desired) that people have finally decided it's easier/faster/less frustrating to just try searching Google for what they want then going to example.com and searching for what they want.