BuzzFeed News:

Internal Facebook emails show how the company used VPN app Onavo, which it later acquired, to closely track WhatsApp’s rise before its $19B acquisition  —  In February 2014, Facebook purchased the messaging service WhatsApp for $19 billion.  The acquisition price was staggering for an app …

Internal Facebook emails show how the company used VPN app Onavo, which it later acquired, to closely track WhatsApp's rise before its B acquisition (BuzzFeed News)
Source: Tech Meme