I am 7 weeks. I have told 4 close friends (and, by extension, their spouses/boyfriends), my sister, and my parents. My husband wants to wait a few weeks before telling his parents, and I'm fine with that. I told my sister she could tell her closest friends so she'd have someone to talk to about it, as long as they promise to keep Facebook silence.
I told my parents they could tell extended family at the end of September, when they'll be visiting our out-of-state family anyway.
Here's what I think for me (I don't think this need apply to anyone else's situation; everyone feels differently and that's valid):
1. The only reason I don't feel like telling everyone immediately is because of the possibility of a loss, which would upset me and also upset the people who love me.
2. A loss can happen at any time -- in or outside of pregnancy and birth. Anyone can be gone, just like that. There's never going to be a "safe" time when I know I can let my guard down. So I decided that, for me (and I can't stress enough that this is a personal choice and all choices are valid), I can't pick a date that is "safe" to tell everyone because I don't feel one exists.
3. If I lose the baby, I would like to have some people who know I was pregnant and will love and support me.
4. If the tables were turned and it was my friend or family member, I would want her to feel she could tell me as early as she wanted, and I would not feel burdened by knowledge of a loss. I would want to support and love her in her time of need. I expect my dearest friends feel the same way.
5. I don't need to tell a lot of people before the end of the first trimester, but I do want to tell a few, close friends, so that I have people to talk to (and so my spouse has people to talk to) about what I'm going through.
6. I'll wait a little longer to tell work (unless I get sick enough it affects my job) and the director of the play I'm in because I don't want the fact of my pregnancy worrying them, but I'll probably tell them early in the second trimester (or when I start to show). I don't think it's important they know early.