SAN FRANCISCO —  Facebook Inc will buy fast-growing mobile-messaging startup WhatsApp for $19 billion in cash and stock, as the world’s largest social network looks for ways to boost its popularity, especially among a younger crowd.

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.

“People are calling them ‘Facebook Nevers,'” said Jeremy Liew, a partner at Lightspeed and an early investor in Snapchat.

WhatsApp is adding about a million users per day, Facebook co-founder and Chief Executive Officer Mark Zuckerberg said on his page on Wednesday.

“WhatsApp will complement our existing chat and messaging services to provide new tools for our community,” he wrote on his Facebook page. “Since WhatsApp and (Facebook) Messenger serve such different and important users, we will continue investing in both.”