There's a reason for this.
There are only so many hours in a day, and only so many of them spent at work. Workplaces will eventually settle on the most efficient tools. These aren't going to be the most powerful tools or even the simplest to use, but the ones who give people the most bang for their buck.
For communications, there's e-mail, IM, and the phone. For document management, there (should be) CMS. For sharing company documents, there's the internal website. For socializing, there's the water cooler.
Where these tools fit in... well, they don't. It takes more effort ("active champions, community managers, and a strategy to nurture") to make them work than the benefits gained over using the aforementioned methods.
Social sites like Facebook work because the links between people are usually physically separated relative to the importance of the communication (the more important the communication, the farther the physical separation). The physical networks are wide and slow, so the digital version has a purpose by making the networks closer and faster. At work, the physical network is close and fast. There's no need for a digital replacement, especially a complex one.
Disclaimer: We also "use" Yammer at work, but the conent is asinine mostly (at least when it's not someone being passive aggressive).