Lot of people and companies are looking for this common feature which is desired for almost every site. I also required such a feature for one on my assignments. Now there are lot of products available in the market which gives you this functionality. Example Improsys . This company offers importing contacts from all streams for 240$.
But i know we always search for free things on net and for this we can use contactgrabber . This project allows one to import contacts from yahoo,gmail,orkut,rediff etc etc.
It is damn easy to integrate this with any website.
I studies the code and found out that this is built on PHP CURL library. I think once anyone master CURL he/she can get anything from internet. He can use the internet as information mine.
So guys PHP CURL is the key. Till the time you learn CURL enjoy this open source project