TFA also says that Austria fined FB ~10M for the same deal.
It might all have been priced into the deal as some other comments have pointed out. If the goal is to truly deter, then the CMA should perhaps make FB unwind the deal at least in the UK.
Also not sure that we can measure the value to Facebook in this way. This is about strengthening Facebook's core business which is worth (just guessing) > $500bn. How much value it adds is unknown but I bet it's a lot more than the acquisition price.
As to the impact on the core business, you're right. However, FB is not worth $500B in the UK alone where this dispute is.
That's not the same as what you've said here.
Seems like everyone is concerned about moral hazards for normal crimes yet white collar crimes like this are extremely underpunished to the point that you can factor that fine into your P&L.