Check out this blog post on Connectedcops.net. If you are an active social media user and you enjoy sharing information with your friends and relatives, one of the useful things you can do while online is inform your community about crime and/or emergency happening in your environment. The whole idea does not cost much and it only includes potential education for users, but the analytics provided by the users could be a big help to law enforcers.
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If you are using social media to alert your community to crime or emergency alerts you should read this