I suppose this may seem obvious, but i hadn't really thought about it. I've heard talk about how people shouldn't post things like "on vacation" on twitter because criminals can use that info to go rob your house. But the idea that people have instantaneous ways to get information and reach out definitely suggests that we could become more efficient in crime avoidance.

Thinking that a great idea for a new service would be some kind of social network 911 call. Press a certain button and automatically post an alert to the following people via twitter, facebook, sms, etc. Then, using GPS, the people could get your message and your location.