Unless you're a teenager, you probably haven't heard of Viber. That's likely about to change. The Tel Aviv, Israel-based messaging app has 664 million users globally. After opening a Boston office this year, it has launched its first big U.S. marketing push, a collaboration of Droga5, Essence and Laundry Service encompassing digital (Spotify ads), print (Fader magazine) and out of home (New York and Boston transit hubs), as well as partnerships with brands and social media influencers. Viber has been such a hit internationally that Japanese online retailer Rakuten last year spent $900 million to acquire it. Also last year, the company tapped marketing chief Mark Hardy, whose focus right now is communicating the brand's revamped image as Viber shifts from being an app best known for offering free international calls to a global lifestyle brand.