google

http://twitter.com/fredwilson

Links are the currency of the internet. Currently the most dominant traffic source is Google.

In terms of passed links Google is remaining about the same, 1% growth per month.
Twitter and Facebook are growing at a fast pace, about 30% to 40% per month.
For the portfolio of sites that he manages 20% of traffic is coming through Twitter and Facebook. If the growth trends remain the same, traffic from Facebook and Twitter is going to surpass Google.

The way Google managed to monetize links coming out of search results, Twitter and Facebook potentially could come up with something similar.