to answer your question, "Why do you think that 75% of the company doesn't use it?" From my experience there are a couple of reasons:
1) No definitive use case - As in the company rolls out Chatter or Yammer and does not embed it into their SOP. They just let it loose in to the wild and hope for the best.
2) No immediate benefits for use - No one has any idea how to truly benefit from it except for a small constituency. Some people use it for communication, some people use it for sharing, some people use it for self promotion. Some people think its "just like twitter". Most just continue using the old school tools that they feel comfortable with.
3) Potentially career limiting - They see only downside to using a tool which potentially communicates with the entire organization and can never be erased.