Common. It is not formal, but not out of place in polite speech. That is you can use it anywhere in spoken language.

Certainly not formal. But still can be used at times even during formal conversations especially when you have to ask lot of questions like [i] How does this work? [/i] or [i] How do you get that task done? [/i] etc.

Hmmm... I guess I'm the only one that thinks "How so?" is more formal. I would use it is talking to my boss if I don't understand the thought process behind what he just said. But I would never use it with my friends or family. I would usually say, "What do you mean?" or "What are you talking about?"

"How so?" can be used in any situation, but take note to the following: It is usually a way of asking for more details. You are essentially asking someone to explain their reasoning or source of information. If is often used to express that you have some doubt about what they told you, which is way trust and body language are important. Friend: Hey, Erkan, what's up? You: Not much. Friend: Did you hear? The boss is giving all of us the day off today. You: (slightly bewildered) How so? Hope this helps. Good luck.