I was under the impression that once you go public, all major decisions have to be voted on by investors. For such a large acquisition, isn't a public company required to let investors vote to give approval?
To put it in perspective, $19B is almost 11% of Facebook's total $117B market cap. That's a huge decision to make on behalf of the shareholders.
I didn't know the answer to this, but decided to do some research. It did strike me as a little strange. There are two things that I found that seem pertinent here.
First, they've come to an agreement, but the deal isn't expected to close until late 2014. That means that a shareholder meeting where a vote could take place may still occur.
But more importantly, it probably doesn't even matter. While Mark Zuckerberg only owns 28% of Facebook after the IPO, their structure is somewhat unique. They have Class A shares and Class B shares. Class B shares have 10 times the voting weight of Class A shares, and Mark Zuckerberg's got enough of them that he has 57% of the votes. All decisions are essentially completely up to him. It's quite possible that there was a vote, and since he had a majority, it could be decided without going on to get other people's decisions.
This situation is pretty unique. Most CEOs have a lot of influence and a lot of shares. This is especially so if the CEO is also a founder. But Facebook is unique in that there aren't really any other large companies where the CEO has complete unilateral control like this.