To say that Facebook replaced MySpace (and therefore, it could also happen to Facebook) is not exactly accurate. Or rather, not the full picture.
https://ourworldindata.org/rise-of-social-media
Facebook surpassed MySpace in monthly user count by April 2008. MySpace hit its peak in December 2008. This wasn't a "migration" from MySpace to Facebook. It was Facebook out growing MySpace.
But that doesn't necessarily mean it could happen again.
"The percentage of US adults who use social media increased from 5% in 2005 to 79% in 2019."
Facebook grew at a time when social media usage was also growing. We don't have a huge, untapped pool of users from which to fuel a "Facebook killer". There was also a significant difference in how the sites we originally used, so there was interest from a lot of early users in using both.
My feeling is that any site that might eventually supplant Facebook as the dominant social media force would need to grow in an underserved market before expanding into the general market, and do it in such a way that Facebook doesn't notice and come crush them first.
What strikes me is that the peak year for MySpace and the year Facebook surpassed it was also the year that smartphones started to take off. Facebook, being the far "simpler" site, would have been much more usable in early smartphone browsers. And smartphones were a huge part of the growth of social media. So it suggests that not only does a Facebook competitor need an untapped market to serve, it needs a platform on which it can create a significantly better experience than what Facebook can do.
Unfortunately, Facebook is ahead of the game on this issue. Why do you think they spend so much money on Oculus and VR? There are leaked memos from Zuckerberg that explicitly spell out this strategy. They are taking over the platform early to avoid ceding the space to a future competitor.