Facebook is acquiring mobile messaging company WhatsApp for about $16 billion, plus $3 billion in stock grants, as the social-networking firm looks to boost the overall time users spend on its properties.

The price tag for the deal — the biggest for Facebook by far — comprises $4 billion in cash and approximately $12 billion worth of Facebook shares. In addition, the agreement provides for an additional $3 billion in restricted stock units to be granted to WhatsApp’s founders and employees that will vest over four years subsequent to closing.

According to the companies, WhatsApp has more than 450 million monthly users, of whom 70% are active on any given day. Facebook had 1.23 billion monthly active users across all platforms as of the end of 2013, with 77% of those using mobile apps.

WhatsApp, which has 55 employees, is on a path to connect 1 billion people, according to Facebook founder and CEO Mark Zuckerberg: “The services that reach that milestone are all incredibly valuable,” he said in announcing the deal.

Facebook shares dropped as much as 4.8% in after-hours trading Wednesday after the deal was announced. The company’s stock price more than doubled in 2013, and is up 25% year-to-date with a closing price of $68.06 per share Wednesday.

Previously, Facebook’s biggest acquisition was Instagram, which the company bought in April 2012 for $1 billion.

The terms of Facebook’s deal for WhatsApp stipulate $2 billion in breakup fees: If the agreement is terminated under certain circumstances — principally related to a failure to obtain required regulatory approvals — Facebook will pay WhatsApp $1 billion in cash plus $1 billion in stock.

The WhatsApp Messenger app is available for iPhone, BlackBerry, Windows Phone, Android and Nokia devices. The company does not sell any advertising; the messaging service is free to use for the first year and then 99 cents per year after that.

Santa Clara, Calif.-based WhatsApp was founded in 2009 by a pair of former Yahoo engineering execs, Jan Koum and Brian Acton. The company had raised $60 million in funding from Sequoia Capital, the Wall Street Journal reported, citing an anonymous source — meaning the venture-capital firm’s stake in WhatsApp is worth some $3 billion.

Facebook was advised by Allen & Company LLC and Weil, Gotshal & Manges LLP; and WhatsApp was advised by Morgan Stanley and Fenwick & West, LLP.