Business networking site experiences?

Right now, a friend is mainly using MerchantCircle, tho she is also signed up for Biznik & maybe LinkdIn. She’s also on Facebook & MySpace for business purposes.
She wants to know how to use them in the most productive ways & how effective they are, so I’d like to hear anyone’s experiences with them & any suggestions for similar sites that might yield good results.

She also wants to know about free ad sites that lets you post lots of ads (like craigslist, backpage, kijiji), including repeated similar ads in various locals OR ads that appear nationwide instead of being limited to various locals (unlike the forementioned sites).