Microsoft was also eyeing WhatsApp, but apparently wasn't willing to pay as much as Facebook for the messaging app.
(Read more: WhatsApp will add voice to messaging service)
"Microsoft was willing to buy it, too. ... I don't know if it was for $19 billion, but the company's extremely valuable," Gates said in the interview.
WhatsApp boasts about 450 million active users, making it very valuable to Facebook, who makes money using user data. The software's reach also has the potential to expand, Gates said.
"It's software; it can morph into a broad set of things—once you're set up communicating with somebody, you're not just going to do text. You're going to do photos, you're going to share documents, you're going to play games together," Gates said.
Google also reportedly offered $10 billion for WhatsApp, but Sundar Pichai, Google senior vice president, denied claims it made such an offer late last month during the Mobile World Congress.