How I would architect a social network product

I've been wondering how I would architect a social network product if I had a blank sheet.

I would first build an entirely local application that monitors the social network of the machines user in isolation to any other network. This would be done by mining data from local email, IM, documents and web browsing. This represents a solution that can be deployed in complete isolation and for the benefit only of the individual on whose machine the product is installed.

A server tier can be added to which is published only metadata for those networks and topics the local user has chosen to share. At this level some mining can take place of backend systems that feed data back to the local agents.

The server tier can be rolled out into further tiers (for increasingly larger parts of the organisation - either geographical or organisation) into which local tiers can leak metadata on networks and topics as dictated by local administrators.