It is difficult. The formality of a letter varies; I wouldn't write to the Minister the same way I would to my parents, or to a close friend. It's the same when speaking in person, really. In my experience, however, this 'network' used by the muggles is often informal to the point of absurdity. I've seen people use incomprehensible slang, and vulgar language not fit for ladies to read, and half of them call me by my first name as if they'd known me since I was an infant.
I don't for a moment suggest that I should behave that way in return. Even muggles at home aren't like that. Perhaps you'll see, when you've been here a little longer.