In the middle of 2009, Brian Acton was the software engineer that no one wanted to hire. Despite a dozen years of experience at Yahoo YHOO -1.27% and Apple AAPL -1.56% Computer, he got turned down by two of the Internet’s brightest stars at the time. First Twitter said no in May. Then Facebook rejected him in August. Today, those snubs are looking like a pair of $3 billion mistakes as Facebook has bought his company Whatsapp for $16 billion.