There are multiple ways to mine relationships... some of which have been largely untapped. One of the big ones, as mentioned by Tim O'Reilly, is the phone network and e-mail. By analyzing incoming and outgoing communication, a user's true network can be mapped out. A recent company profile on TechCrunch discusses a plug-in that works with Outlook Express. The plug-in analyzes messages and message content and maps out a user's network; however this is an enterprise product.
An even larger network can be built upon phone communication - particularly cellular phones. Building a basic Java application that analyzes incoming and outgoing messages, in addition to reporting statistics (perhaps via SMS or text-to-email?), would help build one of the largest real networks in the world. The cellular phone market is huge, just like the e-mail and instant messaging market.
I'm a truly interested in building the largest database of user-to-user relationships and building kick ass products on top... The best part? I want it to be open... I want people to be able to pull their network information (in XML format) and take it somewhere else if they would like.