There are never enough hearts! ❤️ To send lives and ask your friends to gift you lives, you need to be connected to Facebook in the game. Once connected, tap on your Heart ❤️ on the top left corner and choose "Send Lives" to send your friends lives, or choose "Ask Friends" if you need some extra lives yourself.
An envelope will appear next to your Heart ❤️ when friends have sent you lives. 😍