In fact the "market cap reaction" shows us nothing of the sort. A large fine was predicted long ago and has been priced into the stock for many months. The most you can say --- and it's a dubious claim still --- is that the market priced a higher fine in. But you still don't know what Facebook's market cap would have been with no fine, and so don't know how the market values this.

Equally likely, though, is that the market doesn't believe Facebook meaningfully benefited from the actions they've been sanctioned for, and that they'll simply comply going forward, making this fine a one-time event that doesn't need to be factored into Facebook's forward revenues (minus what Facebook could have done productively with the $5B that it lost).