Though the app is still somewhat under the radar in the U.S., it said in December that it has 400 million monthly active users, and Facebook said Wednesday that the figure was now 450 million, 70% of whom are active on any given day. Nineteen billion messages and 600 million photos are sent daily on WhatsApp, said its CEO, Jan Koum, during a conference call Wednesday. Mr. Koum will join Facebook's board. The scale is impressive - five months ago Instagram said it had 150 million monthly active users - but it also means Facebook is now willing to spend $36 per active user to extend its reach among young people. Maintaining high levels of user engagement is key for Facebook as it enters middle age, and messaging apps like WhatsApp, Line, KakaoTalk and Snapchat - which Facebook tried but failed to acquire for $3 billion late last year - are chipping away at its share of people's attention. Get the full story at AdvertisingAge and Reuters