Google Talk has stealthily become a ubiquitous professional networking and business communications tool.
If I've ever had a two-way email conversation with someone in GMail, they show up in my GTalk list. I also see people who are in my G+ circles.
Since then, my Google Talk contacts list has been growing, and is more of a recent representation (but incomplete) representations of my connections than either LinkedIn or Facebook.
Some of these people I've never met. I've talked to some, once or twice about various business ideas. Some are parents of kids on my soccer team. Others appear because we replied to some threads on various discussion lists.
Unlike LinkedIn or Facebook, I didn't have to make a decision to formally "connect" with them in order to maintain a relationship. I just had to send them an email, and have them reply.
I've added a few new contacts on LinkedIn (almost none on Facebook), but there are always new contacts appearing on my GTalk list.
I get to know someone a bit more personally when they are on a chat list. I see when they are online and off, their G+ profile, their GTalk status update. It's also integrated with email, which is my primary communications tool, which makes it frictionless to send them an IM, email or video chat.
It's become more important to me to get people on my GTalk list, rather than on my LinkedIn profile.
How are you using Google Talk for professional networking?