A company's market evaluation has nothing to do with its internal cash flows. Put differently, 15 billion being less than 3% of Facebook's market cap says nothing about how much such a fine would affect Facebook.

Assuming the fine is one-off and market doesn’t devalue facebook because of the fine itself, couldn’t Facebook issue new stock to offset the fine, effectively losing 3% of stock value in the end?

Your question is basically "Couldn't Facebook just ask someone else to pay their fine?" – to which the answer is yes, they certainly could. But I highly doubt it'd work. I mean investors would basically be bailing out Facebook and wouldn't get anything in return.

Look at it from the perspective of existing investors: They would be forced to buy the new shares Fb is issuing in order to maintain their respective percentage of shares and, thus, their expected future dividends. There's nothing to gain from that – in fact, their investment's expectation value just goes down as they now need to pay an additional price for the same expected future return. Needless to say, shareholders don't value such a move, so any CEO trying to pull off such a thing would likely be removed immediately.

>There's nothing to gain from that

What about not getting shut down for failing to pay a fine?

I was talking about the investors not gaining anything from the company issuing new stock in order to pay fines in general. (Which also explains why this is not a common practice.) Fines that are threatening a company's existence are certainly a different beast as the investors' expectation value calculation then needs to factor in that if the company goes bankrupt they lose their entire investment. So refinancing the company by buying new stock then becomes more attractive because it will (or might) keep a shareholder's overall expectation value at least slightly above zero.

But even in these situations, I think a case can be made for why refinancing the company by having it take a loan is still more attractive. While from the perspective of an existing shareholder it will reduce future dividends in a similar way as new stock, there will at least be no opportunity costs. (If the company issues new stock, the investor has to consider what else they could do with their money instead of throwing fresh money at the company.)

Of course it does. They can easily convert 3% of their market cap into cash via bonds or issuing new stock. If the fine we're 150% of Facebook's market cap they wouldn't be able to.

I don't think it is as easy as you portrait it to be. (See my response to your comment's sibling.)