It would be weird that they'd make it display a 404 though. It's possible Signal could be returning that when they see FB perform a request to them for some reason (e.g. want to make sure no metadata leaks to FB).
Whatsapp isn't close to a monopoly in the US. Here iMessage, SMS, Snapchat, and Facebook Messenger by far have greater market share. You can argue that FB Messenger should be linked to Whatsapp for the purposes of anritrust, but even then, there is very healthy competition.
Monopoly is one part of antitrust laws but you don't have to be a monopoly to be breaking antitrust laws. This is pretty clearly anti competitive. Other examples would be price fixing or bid rigging (where you purposefully underbid so your competitor can win the contract for cheaper. And then your competitor does you a favor later).
No more than a Ford dealer buying a billboard outside a Chevy dealership. It's not anti-competitive to advertise in the same place your competition advertises.
The issue with Facebook doing this is the fact that they're a huge company that is buying these ads where the smaller company cannot.
It is illegal and anticompetitive to be a huge company using enormous resources to squash competition for smaller firms. There are many ways in which this is done. Not all of them are illegal, depending on how you look at it.
My opinion: Buying marketshare is usually bad for the user.
At the very least, they shouldn't be allowed to advertise for the words "Local Craft Soda (tm)" -- especially if competitor's name is what they are buying.
I can also say by having lived there that it is the same for Germany, Switzerland, Spain and Italy. There will always be an exception to the norm, but it is clear that Whatsapp is the number one tool for instant messaging/calling.
Facebook and Whatsapp are effectively the same entity, as the recent requirement of sharing Whatsapp data with Facebook shows. It's basically Microsoft with Windows and IE using special hidden dlls to execute faster than Netscape and that was of course a landmark and exemplary example of anti-trust abuse by a non-monopoly in either OS or browsers. Facebook needs to be broken up and the special ownership rules privileging Mark Zuckerberg need to be removed.