Is it the first pregnancy?
The chance of a miscarriage goes down by more than 60% after the first trimester. Most first pregnancies result in a miscarriage. It's a lot easier to deal with it privately than it is to jump on facebook and tell everyone you know that she's not pregnant anymore. It creates a lot of emotional stress for her.
My wife had two miscarriages. One very privately (just our parents knew) and one very publicly. The second was miserable because everyone wants to tell you how sorry they are and they keep reminding you what happened for months.
You should tell people who need to know (parents, immediate family) and then wait until you know things are progressing well, at least past the 10-12 week mark. If something happens, it's a lot easier to deal with.