When it comes to messaging, Snapchat and Facebook-owned WhatsApp and Messenger are the go-to apps of millennials and marketers alike. But increasingly, a new crop of messaging apps is growing in popularity. Players like WeChat, Line, Kik and Viber are beginning to quietly capture the attention of brand marketers via sponsored stickers and chat bots reaching millions of millennials and teens. "Messaging today is what Facebook was in about 2006," said Evan Wray, co-founder and vp of Swyft Media, which builds messaging-based campaigns for advertisers like L'Oréal and Coors Light. "The tricky part is that every single app is different and [also] a new environment for brands. They're each trying to differentiate themselves slightly." Here, what marketers need to know about four hot messaging apps in the coming year.