For a hefty $40M, and IMHO a brilliant investment, Facebook has acquired some patents from Friendster. Among them:
"A System, Method and Apparatus for Connecting Users in an Online Computer System Based on Their Relationships within Social Networks."
"Method of Inducing Content Uploads in a Social Network"
"Authorization and authentication based on an individual's social network"
"System and Method for Managing Connections in an Online Social Network"
"Compatibility scoring of users in a social network"
... 18 in all.
As Google rallies its resources for another stab at social networking, acquiring these patents is a smart defensive move for Facebook.