$ 0 0 The acquisition of the hot messaging service with more than 450 million users around the world stunned many Silicon Valley observers with its lofty price tag. But it underscores Facebook's determination to win the market for messaging, an indispensable utility in a mobile era. Combining text messaging and social networking, messaging apps provide a quick way for smartphone users to trade everything from brief texts to flirtatious pictures to YouTube clips - bypassing the need to pay wireless carriers for messaging services. And it helps Facebook tap teens who will eschew the mainstream social networks and prefer WhatsApp and rivals such as Line and WeChat, which have exploded in size as mobile messaging takes off.