Thank you all for your constructive ideas. I just wanted to let you know what I have chosen, how I imagine it to work in my system and why it is this way. After looking at everything you mentioned, I think there are basically only two different broad skills needed:
Empathy : This is your skill to observe, analyse and identify what the other person's all about. It is the skill to gain information, understand the words between the lines, sense the other characters motives. It's the social skill you need to have something coming in. Includes: sense motive, gather information, diplomacy (if you want to get information or just positive relations)
Acting : This is the skill to present your opinion in a clear and convincing way, to persuade, confuse or be entertaining; it's the social skill to get something out. Includes: bluff, perform, entertain (talking), intimidate, diplomacy (if you want something to happen).
Explanation: The third skill would in my humble opinion always be some combination of the other two. Both skills are usually used against each other.
Example: If you want to fast talk someone into something, it's Acting, to resist, the person uses the Empathy skill. If you want to get some information from some guy who doesn't trust you, you use Empathy against his Acting. Intimidation is a bit special here, as you can intimidate someone, and the only outcome is that he's intimidated if his Acting fails (using Acting as a defense as he can hide if he's intimidated; if he is or not is ). To find out if the lie he presents you then is true, you still have to use Empathy against his Acting.
Wording: The sensing skill is called "Empathy" because it's very broad, even if empathy is also a part of acting in a way, as the actor usually needs to hit a nerve for his audience, and therefore understand this. BUT I think it's okay to subsume "acting hits a nerve in audience" in the skill of Acting.
Weighting and skill level in the beginning: As there are only these two social skills, and as they have "offensive" or practical effects as well as defensive effects and I think they're pretty important, I would let them be pretty hard to learn (using a learning factor that multiplies the needed XP for the next level) and begin on a relatively low level for humans, so that characters have a chance to be bad at this when they spent their time in isolation or so. NPCs will have average skill levels in this.