"Facebook is worth nearly 1/2 trillion dollars."
What does "worth" mean? If you mean money in the bank you might have a point although the shareholders might want some of that "returned" to them.
If you mean stock market valuation then you have to account for the fickleness of the stock market. It's not real money, just a projection of the price at which recent transactions took place. A surplus of sellers over buyers will change that in an instant and feedback can amplify such changes. Sticking with your point of GDPR fines a 4% fine would cause at least a bit of a wobble. If there were a few cases ongoing and FB had to announce it was making a provision of an eighth of its turnover for possible fines the feedback loop might really start a plunge.
Actually, I was thinking not of GDPR which is done and dusted as far as legislation is concerned. But what of new legislation? A reconsideration of corporate taxation rules? What about the idea of having companies pay the data subjects for slurpage as discussed here https://www.theregister.co.uk/2018/06/22/cowen_technopoly/ ? Or, more likely, governments being governments, valuing the slurpage as a taxable asset?
And don't just think in terms of fines; we are starting to see criminal sanctions against company officers, e.g. in the case of the VW emissions business. Zuck and his friends really wouldn't like legislation that puts them in line for those (actually GDPR already does).