You kinda got it right. Corporate social networks are there to promote the distribution of information. But for God's sake, do NOT call them "Facebook for the Enterprise", "Enterprise Social Network", or anything like it. Do not mention enterprise, do not mention facebook, twitter, instagram, or any other idiotic time killer. Call it what you want - "Acme's place for Engineers to comment on feature update requests from PMs", "Worldwide Information Sharing Platform", just don't use the Facebook analogy.
Second, employees are asking for tools to better share information. Sharing your latest beer run or cake social pictures is not information. Sales people want to know if someone knows someone at corp A, where they just got a meeting for. Support people want to know if anyone has seen weird behavior x that isn't documented anywhere, and hasn't been tagged in a case yet. Others want to know if there are some good presentations on a topic so that they don't have to create them by hand, or just want to get in touch with someone in a particular position but who they have never met. An enterprise social network helps that.
Here's the third issue, and this is where most corporate social networks fall down. It has to be used by the execs, and the execs have to show to everyone how to use it right. If they start posting pictures of their latest executive retreat where everyone has a Margarita in hand, or they start to talk about what movie they saw over the weekend, shit will irretrievably go in the shitter. Lack of adoption of a corporate social network is always and every time the fault of the corporate leaders. Whether the execs, or just the people everyone wants to listen to.