Facebook's Safety Check feature is a good way to tell your friends and family that you're alright if you're in the midst of a disaster like Hurricane Harvey. You can find the Safety Check in the "Explore" section of the mobile app. You can get to it by tapping the hamburger menu on the bottom right corner. Once you tap into Safety Check, you'll see the current Safety Checks around the world or you can create a Safety Check. Then you can see people that are marked safe, and mark yourself safe if you're in an affected area.