I don't see why social attacks could not be performed in combat.
Like... deceit roll against a powerful enemy while taking cover, "You'd better leave fast if you want to live. I have about 20 people coming on the way right now!"
Definitely seems appropriate to me.
On the contrary, I don't see why social attacks need to be included in physical combat. It seems like an unnecessary complication.
I would treat that Deceit roll as a maneuver to place the aspect "He's Got Backup Coming?!" on the enemy. The aspect could then be tagged or invoked as normal, either for the normal +2 or as a compel to get the enemy to retreat (and getting the enemy to withdraw seems like the objective of such a tactic in the first place). That seems simpler and more elegant than dealing with the social stress track.
The book also seems to imply that social combat is an entirely different beast in a way that mental combat isn't. Social combat has its own method of determining initiative (Empathy, rather than Alertness), and there are no powers that deal social stress in the way that Incite Emotion deals mental stress. It seems like aside from those rare, exclusively mental conflicts that happen every so often, mental attacks/stress are a subset of physical combat, while social combat is a different method of conflict resolution.
Plus, if an enemy could be Taken Out of a physical combat by dealing social stress, you could create a character who wins firefights
just by hurling insults, which doesn't seem to make any sense. Working an enemy into a blind, frothing rage is a time-honored way of setting them up for the killing blow, but this can be accomplished by stacking maneuvers with social skills and tagging them all for a physical attack. Dealing social stress just doesn't seem appropriate in a physical combat situation.
Edit: UmbraLux makes good points too. Social combat seems to take place at a different speed than physical combat.