I'm using it at a 100-person office but nobody wants to integrate yet-another-step into their daily processes or have to hop around different places to get different pieces of information (because naturally, John Doe will always want to do it his way if there isn't a policy otherwise or when it becomes too chaotic). It's the same 5-10 of us that make irregular posts and maybe an additional 5 that will occasionally lurk when they remember to. I suspect those people use the desktop client and just always have it running. No one takes advantage of the chat, the secret groups (okay, maybe one team is that I'm not aware of), or the file-sharing. That said, I've complained about the shoddy design and the confusing interface here before, so I'm happy it didn't take off internally.
This has led me to believe that if they're not able to use social features from within the software they're already using (that's already connected to our existing data), they probably won't use them to any effective capacity at all. We run a homebrew CRM, but I don't think slapping a chat or a file system into it will suddenly fix the problem. That said, I think company culture has a lot to do with it too; getting up and talking to people and scheduling meetings with them is highly encouraged here.
That all said, Jive's business enterprise stuff is probably the best in this arena right now because of how much you can do with it and how extensible it is, but I've been asking for us to switch for two years to no avail because management doesn't see the value in constant, but passive, streams of information.