Amber alerts, flood watches, escaped prisoners, earthquakes and other public service messages have been part of Twitter since 2013.

Twitter Alerts are messages sent during an emergency or crisis by public agencies or emergency organizations. When a Twitter user follows an account that is a part of Twitter Alerts, emergency notifications sent by that account will be marked with an orange bell. These messages will include the hashtag #alert. 

Twitter users can choose to receive a text message or push notification via the Twitter app when alerts are sent by an organization. That set-up is done by signing up on an account's alert page:[username]/alerts.

Twitter launched its alert system in September 2013 with several partners in the United States, Japan and Korea. The project has since expanded to Spain, Brazil, Australia, the United Kingdom and Ireland and includes global nonprofits such as the Salvation Army. Here's the full list of participating organizations. 

Facebook joined Amber Alerts, the U.S. notification system for missing or abducted children, in January 2015. The feature expanded to Canada in May.