Briefly, I need some advice on a solution that I’m trying to implement. I'm implementing a social network, using .net technologies and I need to infer knowledge from the data collected and persisted at the database (A kind of Graph/Link Mining). My questions are:
I'm not a BI expert but yes this does sound like a BI tool could be useful.
I don't know of any .Net library, but CodePlex is usually a good place to look if you're after something open source.
Using a separate database is definitely a good idea if you want to do lots of reporting and the Transactional part of the application is usually busy. FYI transactional databases are usually known as OLTP where-as databases intended for reporting are usually classified as OLAP; the Wikipedia links give a fairly good introduction if the terms are new to you.