Facebook, the world’s largest social network, agreed to acquire mobile-messaging startup WhatsApp for as much as $19 billion in cash and stock, seeking to expand its reach among users on mobile devices.

The purchase would be the biggest Internet deal since Time Warner’s $124 billion merger with AOL in 2001, according to data compiled by Bloomberg. The accord includes $12 billion in stock, $4 billion in cash and $3 billion in restricted shares, Facebook said Wednesday in a statement. WhatsApp has more than 450 million members, with 1 million users being added daily, Facebook said.

Facebook, which acquired photo-sharing service Instagram for about $700 million in 2012, is counting on applications beyond its main social network to reach more users on smartphones and tablets. WhatsApp competes with Snapchat Inc., which rebuffed a $3 billion offer from Facebook last year.

“Facebook is clearly taking out one of its main competitors,” wrote Paul Sweeney, a Bloomberg Industries analyst, in an email. “They are buying 450 million loyal users and an extraordinary growth story, but at a staggering cost.”

Mountain View, Calif.-based WhatsApp, lets users send messages through its service on mobile devices based on different operating systems including Apple’s iOS, Google’s Android, Microsoft’s Windows Phone and BlackBerry’s software.

Unlike traditional text messages, which consumers pay for through their mobile-phone plans, WhatsApp is free for the first year and 99 cents a year after that.