There definitely was a big difference between MySpace and the other social networks. Facebook ran well and worked. People forget the total shit show MySpace was in the middle of 2008. The site ran terribly, was getting hammered by spammers, and they starting covering it in banner ads. We didn't see a repeat of those problems with Instagram or Snapchat.
There is a coolness factor. It isn't as defined as fashion, or the latest hot nightclub, but it is there. That alone won't be enough to make the "next" Facebook, but I think it is the foot that gets stuck in the door.
Facebook might be able to acquire the next challenger in the US, but they will definitely fail to get it by EU regulators.
Networks are driven by positive feedback both going up and down. This sounds good, but isn't: the system and balance points are inherently unstable. Nothing succeeds like success, or fails like failure.
Nevertheless, your larger point holds, Facebook users are even less cool now than they were before.
But it does mean that an even somewhat diligent antitrust enforcement could strangle them to death. They shouldn't be allowed to acquire their future competitors. If the US won't stop them, maybe European regulators can?
What I mean is that we are comparing two different beasts, so I'm not sure "it happened to MySpace" is a good telltale sign of what will eventually happen to Facebook.
I've seen this a lot more in countries where internet access wasn't too common until the past 5-10 years and people didn't start out with a less centralized web before apps and closed networks gained popularity.
My partner isn't from the US originally and when I mention how obnoxious it is that Facebook is like the new AOL and I thought we were past this, she reminds me that it's all anyone back home uses for anything and they didn't have internet access back then.
To her and her friends/family back home, the internet basically is Facebook (and occasionally being forced to open their browser app to search for something if they don't just ask around on Facebook). A handful of other apps and defaults define the internet for them and anything else just sounds like too much hassle.
I suspect Facebook knows this and will keep acquiring new platforms when they can.