I GM DFRPG too, and I have had similar problems.
The first thing I did was remind the PCs that they can, and should use maneuvers. Also declarations. Fortunately one of my characters is a social monster and the other two were more than happy to pass her tags rather than try to take shots themselves.
The second thing, and this might not work for your group, is to remember that social combat can be every bit as abstract as physical combat. We've found that the best way to handle it is to start with actual conversation but if it eventually moves into "I'm using False Face to try to give him the impression that I'm less reasonable than I am by glowering and making veiled threats" that's fine. None of my players has a Deception of Superb in real life.
Third, keep in mind what the NPCs are going for, essentially what Taken Out condition they'll want to impose on your PCs if they win, and what the PCs have to gain from victory. It's helped me structure the dialogue for the NPCs and figure out when to start talking concession.
Finally, don't be afraid to over act. I am not an actor, so to make NPCs memorable I tend to overdo their more obvious characteristics. If you can pull off subtle, great, go for it, but I've found that the unsubtle NPCs are the ones that stick with them. They just love Marcel Raith (think Pepé Le Pew), and literally demanded that he make repeat appearances.