By Jeremy Wagstaff Singapore (Reuters) - WhatsApp may be hugely popular but its forays into Asia, the world's biggest mobile market, have had mixed success, raising questions about whether it can sustain the explosive growth Facebook Inc cited to justify its $19 billion price tag. Data from app metric company App Annie, for example, shows that WhatsApp ranks as the top communications app in only three of 13 Asian countries tracked - Hong Kong, India and Singapore. "WhatsApp has been a strong player in Asia, but in the past year has faced strong competition from LINE and WeChat," said Neha Dharia, India-based analyst for Ovum, a technology consultancy. "WhatsApp has not been displaced by these players, but has seen stiff competition in growing its market share." Facebook said on Wednesday it would buy WhatsApp for $19 billion in cash and stock, in a deal worth more than Facebook raised in its own IPO.