This is not a sustainable amount to be fined. I think they made a net income around 20 billion last year, this a quarter of that. A couple more of these fines and that's a really dangerous situation - companies at Facebook's stage are valued on multiples of earnings with growth, not revenues, and 75 to 50 percent reduction in earnings wiped a lot of shareholder value off the table, meaning the shareholders (basically just zuck) would need to think twice before committing these violations again.
Except it didn't whipe a lot of shareholder value off the table, the stock rose by 5% after fines were announced. Zuck himself made a cool billion off the bump.
News like this is already priced in and the market reaction is an irrational mix of random expectations. Some would have thought it would be more, some will think that this the end of Facebook's regulatory troubles, some will think that people overreacted over the last few days and are buying now that the uncertainty of the fine is gone and we know how the settlement stands. If this was a surprise 5 billion, then the stock would have dropped by way more than the 25% you might expect because of panic. You also have the growth story.
A good way to put it is that the stock price would not have changed by more than a few percent today anyway, but in a world with no fine the starting price would have been higher. And in a world where we see more and more 5 billion dollar fines, they'll be less and less shrugged off as one offs even without increasing penalties.
A larger fine was priced in because investors were uncertain of earlier estimates of the fine being accurate. When the settlement news was announced, the stock bumped because it was better than expectations.
