WhatsApp is a cross-platform instant messaging service launched in 2009, founded by former Yahoo! employees Brian Acton and Jan Koum. It gained popularity due to its ease of use, affordability, and cross-platform compatibility, amassing 450 million monthly users within a short period. Facebook acquired WhatsApp for $19 billion in 2014, marking one of the largest acquisitions in tech history, surpassing deals made by major companies like Google and Microsoft.