I'm a "you have to know the rules to break them" kind of guy, so I'm with TAC and CC here all the way. Most of my daughter's friends refer to me as "Mr. F*******" (my last name). My daughter refers to most of her friends parents as "Mr./Mrs. So-and-so". There are select exceptions, though. But for me, and maybe this has to do with having a daughter, I see no need to be overly familiar. I'm not looking to be their best friend. (Side bar, but I have a personal, unspoken rule that I make it a point to not even be alone with any of my daughter's friends unless it's an absolute emergency; I just don't want to be in that position).
With my two oldest step-kids - one's 26, one's 21 - it's a little different. The oldest, while I'm still not looking to be his (or his friends) drinking buddy, I'm more apt to use my first name. We're getting there with my second-oldest (though it's my step-daughter, and I have a similar, personal unspoken rule there too.)