From December 2007 to December 2008, social networking website have expanded their user base by 13 percent, according to Comscore. Facebook showed an exceptional performance increase--in that period, Facebook experienced a 57 percent gain. Part of Facebook's strength (in the eyes of the advertising and SEO community) is their "word of mouth" referral style that provides a huge traffic boost for sites like PerezHilton.com and Cafemom.com.
Ultimately, Facebook and other social sites like Twitter are emerging as a serious threat to Google and algorithmic-based search engines.