Don't know about Yammer, but Valve has a very flat structure where engineers work directly on projects that they are interested in (and presumably that they think will bring in revenue). Quote from their (definitely must-read) employee handbook:
"We’ve heard that other companies have people allocate a percentage of their time to self-directed projects. At Valve, that percentage is 100."
"We’ve heard that other companies have people allocate a percentage of their time to self-directed projects. At Valve, that percentage is 100."
http://www.valvesoftware.com/company/Valve_Handbook_LowRes.p...