Once I got over my initial enthusiasm with social networking sites, I started saying a simple phrase: social networking is a feature, not a business plan.
From Marc, I see that Reid Hoffman's LinkedIn has jumped full force into jobs. The features? Relationship (follow the connections between you and potential employees/employers), References (people connected to you can give references), and Reputation (displays endorsements for employees/employers).
Great stuff, and the latest graphical updates have made LinkedIn nice looking and very usable.
LinkedIn is the one network that I've stuck with. Many of my earlier complaints about it were that "there's nothing to do there". Well, that's the point. It sits there quietly waiting for the time when you need to search through your business connections.
And maybe one of these days they'll support Canadian ZIP codes. :P