In any case I don't think it is about the calculation or the final number. It's really the approach that you develop that's interesting.
I've seen this mostly used in (management)consulting though, where you actually are regularly expected to solve problems that you might not have deep understanding of. When I was interviewing developers I discussed how they would architect an application to solve a specific problem. It gives similar insight and is closer to the actual job.
The reason is that to many people (myself included) your answer above is nonsensical.
“With ~50M people capable of clicking ads, and assuming they click one ad a day at 20cnt CPC, it would be around 3.6bn. I think my assumptions are on the high side, but order of magnitude is probably correct”
Where did the "people capable of clicking ads" 50M estimate come from? Why one ad a day on average? Why not 0.2? Or 15? Why 20c CPC and not 0.001 euro cent? Why not 1USD?I'm all for "first-principals based structured thinking" but asking someone to demonstrate that by generating a rudimentary formula, that won't survive any contact with reality, and populate it with almost completely random values, seems like a weird way to go about it.
E.g. someone who guesses "$3 billion" on only a gut feeling likely wouldn't be a strong hire even if the number is correct, while someone who can correctly decompose the final answer into its components and make reasonable estimates would be a much better candidate.
For estimation interview questions, and problem-solving questions more generally, a "structural" approach to breaking down problems is important because on the job the interviewee will need to be able to identify what levers are available to them.