Yahoo One Connect is an ambitious, and very good looking app. Unfortunately it is close to useless for most people.
The idea of the app is to bring all of your contacts in one place: and by "all" they mean the following social networks and services: Facebook, Myspace, Bebo, Friendster, Twitter, YouTube, LastFM and Flickr. No support for gmail, Linkedin, hotmail, or any of the other of hundreds of other "social nets" as Yahoo calls them.
The problem is that even when people use multiple services, they use them differently, and the data doesn't easily mesh together. I have twitter friends, but i don't have their phone numbers, and cross referencing them with Yahoo mail doesn't help. What it does create is a mess of old and stale contacts, mainly without phone numbers or ability to im.
The Twitter/Facebook feed integration is pretty but clumsy. People who twitter will use a dedicated twitter client like Twinkle instead.