As I've embarked in an interesting journey about creating a social networking set of tools and community site for my company, I tend to look more closely on how others did it and the dos and don't on the topic.
Here is an interesting quick summary of the considered two ways social computing are adopted in the enterprise, according to Dion Hinchcliffe: Top-down and bottom-up. No rocket science here, but it's interesting to see first that both ways are recommended and not exclusive as well as what drives and supports each way.
Here is an "encouraging" note to those of you in a hurry ;-) from Social Computing Journal :
"Based on their findings, the Nielsen Norman Group estimates a timeline of approximately three to five years for most organizations to successfully adopt and integrate social technologies into their intranets. They also suggest that the political and cultural changes needed for its useful and widespread use may take longer"
Have fun! I'll keep you posted on how it goes for us.