yeah, but why doesn't it respond with 404 as would be appropriate?

"You want to see a page we don't have? Let me show you some ads instead!"

Technically, a 404 page with ads on would be fine too ..

Most web sites don't 404 when fed unrecognized get params.

While that's true, I would kind of expect Facebook to do better than most sites. Oh, well.

Facebook tries to do whatever maximizes engagement. Technical correctness only matters when it serves that primary goal.

Some times a benefit can be indirect and hard to see immediately. Being "technical correct" reduces complexity, which again reduces costs down the road. For example, it makes it easier for third parties to integrate with your service, to name one benefit.

It may never be clear for each individual feature, but violations compound to form a mess of unpredictability. Facebook generally appears to me as being a company with a very strong engineering culture and so it surprises me a bit that they would let something like this slip. Maybe I'm just not seeing the whole picture and it is a clearly thought-out tradeoff and not simply negligence.