Brian Acton will now finally join Facebook, which rejected him a job four and a half years ago. In a tweet on August 4, 2009, he wrote, "Facebook turned me down. It was a great opportunity to connect with some fantastic people. Looking forward to life's next adventure."

This rejection came just a few months after he was turned down by the other rising social networking star, Twitter. In a tweet on May 24, 2009, Acton writes: "Got denied by Twitter HQ. That's ok. Would have been a long commute."

The acquisition of the hot messaging service with more than 450 million users around the world stunned many Silicon Valley observers with its lofty price tag.

But it underscores Facebook's determination to win the market for messaging, an indispensable utility in a mobile era.