The bulk my friends on FB are people I attended school with. Some of Curly's relatives whom I've never met are also in my group, plus my sister and sister-in-law (though my daughter is on FB, she's not on my friends list, yet her mother is). Then I have Burner peeps, Pagan peeps, drum corps peeps, and some folks left over from my first batch of friends on FB.
When I first joined, I found a group and contributed to it right away. The founder of the group was pleasantly surprised (it was a music group with few formally-trained musicians on it) by me offering up a bunch of information out of the blue. He made me an officer of the group and directed people from his network to friend me. I had a group of 70 or so friends pop up--musicians from around the world. Once I got my feet under me, I pared most of them from my friends list, though a few remain that I've had direct interaction with online.
I'm not terribly picky about adding folks to the FB friends list. I don't blog anything extremely personal there. I blog (and repost it) on other sites, so my friends lists on those sites are much smaller. On Tribe or LJ, my standards are much different for adding folks to my list, for that's where my blog appears with all sorts of juicy stuff in it--whether debuting art or reporting medical results (and how unemployment is literally killing me slowly) or how I processed the breakup with L all those months ago.
The peeps on those sites all know I'm poly and Pagan, along with hearing about the drum corps, the artwork, the other hobbies, and so on and so forth. Some of the folks on FB do, most don't. At least one has figured out Curly and I are poly from my profile, though I doubt most would bother to put two and two together.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.