Facebook Inc. 's $19 billion deal for WhatsApp in part is a move to bolster the U.S. company's position abroad.

But in Asia—which has the world's largest, and possibly most avid, social-media audience—Facebook still has its work cut out for it.

That is because in Asia, even more than on Facebook's home turf, the big, growing social-media market is on mobile phones. And if Facebook wants to be as dominant on smartphones in Asia as it has been on personal computers, WhatsApp will need to lure users away from three popular apps in the region: Naver Corp. 's Line, Tencent Holdings Ltd. 's WeChat and Kakao Corp.'s Kakao Talk.

Each, though little known in the U.S., has a stranglehold on a big Asian market: Line in Japan, WeChat in China and Kakao in South Korea. Those apps, which offer social-networking and entertainment features on top of instant messaging, represent trends in mobile Internet that started in Asia, a shift from trends starting in the U.S. and then spreading to Asia.

While WhatsApp is popular in some Asian markets—such as Hong Kong and Singapore—Line, WeChat and Kakao have expanded in the region over the past two years, creating a tough environment.