Menlo Park-based Facebook said Wednesday that it will pay $16 billion for WhatsApp, a developer of mobile messaging apps, splut between $4 billion in cash and $12 billion in Facebook stock. According to Facebook, the deal will help it accelerate its growth and user engagement. WhatsApp develops a real time, mobile messaging service with around 450 million monthly users. WhatsApp is led by Jan Koum. Facebook said it was advised by Allen & Company and Weil, Gotshal & Manges LLP in the deal; WhatsApp was advised by Morgan Stanley and Fenwick & West, LLP. The deal provides an extraordinary return for Sequoia Capital, which had invested $8M in WhatsApp back in 2011.