Facebook is encroaching Google’s prize turf: search. More and more, people you know and trust are directing online traffic, as opposed to clever algorithms. Clearly, search is going social.

In one area it has already caught up. Facebook is now the top source for traffic to major portals like Yahoo, AOL and MSN. According to a recent study by web measurement firm Compete Inc. Facebook now accounts for 13% of the traffic to major portals, compared to 7% for Google.

According to Hitwise, Facebook is also becoming more enticing for publishers. Facebook recently beat Google News in upstream visits to news and other websites by a margin of better than two-to-one. This time last year they were both about even. Since then Google has plateaued while Facebook has shot up.

To be sure, Google is still the commanding leader in search, accounting for two thirds of all searches made in the U.S. But even here Facebook is doing well. It posted a 13% month-on-month gain searches in January.

Clearly our web surfing habits are changing, based increasingly on referrals from our trusted social network. Businesses, whether it be marketing or dealing with customer relations, must now take into account this phenomenon of the growing importance of “peer-to-peer influence.” Call it a shift from search-engine optimization to social-media optimization.

In other words, the net keeps looking more and more like a person than a machine.