I took a lot of cues from the LGBT community in dealing with it in terms of friends. I basically just came right out and said that I had made a significant personal discovery, I was poly, and let everything else fall where it did.
The vast majority of my friends were alright with it, some of them weren't surprised. I think I was actually more surprised at how neutral the reception was. People were supportive but no one lost their shit. I had expected at least one person to get strange but nobody did. I actually am yet to have anyone actually get really and truly bent out of shape over it. That was actually the most surprising thing; that nobody really got worked up in any respect. I had kind of prepared for a showdown that never came.
Family is a different matter. They dont know mostly because while they're all very "live and let live" on most issues, normality is a big deal. Appearing and acting "normal" is very important to most of my immediate family and they're not great at dealing with people who fall outside the societal normal. My brother in law also hit the Jesus juice pretty hard and as a result, I worry that if he found out that I'd be denied the ability to see my nieces and nephews whom I'm probably closest to out of anyone else in the family.
Family makes it awkward though. My mother will groan and sigh, asking when I'm going to get married and have kids (she knows about one of my ladyfriends, has no idea about the others). I've tried to prime her for the idea but I think its just too much of an out-of-context problem for her to consider. My father I could care less. He'll make a group sex joke and then ignore it entirely. If it doesn't interfere with what he does, he doesn't care.
From what I've seen, I think most people over-estimate how negative the reaction of people around them will be and there's one or two people whose reactions completely blindside them for good or for ill.
I am as direct as a T-Rex with 'roid rage and about as subtle. It isn't intended to cause upset, I just prefer to talk plain. There are plenty of other people here who do the nice, polite thing much better than I can. I'm what you'd call a "problem dinner guest."