I actually just learned today that Riot (the IM app) is not related to Riot (makers of hugely popular video game League of Legends). I thought the occasional mentions I was seeing of "Riot chat" meant that LoL's mobile chat client was gaining traction among people who don't play the game.
It's far from perfect, but it's better than any other option for most. Go to the android/apple store, click instlal, start chatting with anyone in your address book that uses it.
Anything else is either much less secure, or much harder to install/use.