Social skills are not mind control. They affect people's perceptions, but they shouldn't be able to overwrite their most deeply ingrained personality traits. For example, if they are dealing with someone who is the only source for something, and he also happens to be a curmudgeon with a sour disposition, who shopped around for the cheapest rest home to put his parents in, he is not going to suddenly bend over backwards to give the face what he wants at a bargain price. At best, he might grudgingly get him to give him the price he gives his regular customers. You need to set some limits on social skills, no matter how ludicrously high they can get, because otherwise you will wind up with one player making your NPCs and all of the other PCs into his puppets. No fun for anyone (except maybe the player of the face).
Social skills can also run into hard limits that are externally imposed on the NPCs. Maybe the fixer literally can't afford to let the item go for that price - he would be selling it for a loss. Johnsons have a hard limit to how much they can offer the team for the job. However - social skills, like combat skills, can be improved with tactics. Maybe all of the smooth talking in the world won't get the boss's secretary to betray him, or get that guard to open the door; but what if you convince the secretary that you are the boss's friend and need to see him because it's an emergency, or lure the guard into the cell by feigning a medical emergency? A street samurai with 20 dice in pistols can get some things done simply with his high dice pool, but if he knows when to run for cover, when to make called shots, and other elementary tactics, his odds of success improve dramatically. Playing a high dice pool face should be the same way.
Finally, and this applies to usage of all skills - it needs a modicrum of plausibility to work. The face should not be able to make a Human Nation agitator see the error of his ways in one talk, or turn a frigid lesbian straight. And for social skills to work at all, the face needs to actually be negotiating, or intimidating someone, and so on. If the face is being an asshat, give him that rules-mandated etiquette check to notice he is about to commit a social gaffe. If he ignores the warning, have him suffer the consequences. Be sure NOT to simply give him a negative modifier for things like that - he will simply roll his umpteen remaining dice, and you will have an implausible result to somehow justify.