First let's assume for the sake of argument that you're right: it still doesn't matter, Facebook is not the government. Facebook can tell stupid people to shut up and stop putting innocent people at risk and that is not breaking up "peaceful and legal assemblies" as you put it.
Now, going back to the government breaking up "peaceful and legal assemblies" (which these are not), it still doesn't matter: you are not guaranteed every right in the constitution at all times equally. A good example of this is time, place and manner restrictions: the government can't tell you that you're not allowed to advocate for a candidate for office, that would violate the first amendment. However, they are allowed to tell you that you can't advocate for a candidate for office at 1AM with a loudspeaker in a residential neighborhood. That's effectively what these people are doing: there is a pandemic, their actions could hurt a lot of people, so their stupid assembly is not legal. (Disclaimer: I am not a lawyer and nothing I say constitutes legal advice)
Also:
> In addition to serving as medical functions, isolation and quarantine also are “police power” functions, derived from the right of the state to take action affecting individuals for the benefit of society.
https://www.cdc.gov/quarantine/specificlawsregulations.html
https://www.cdc.gov/quarantine/aboutlawsregulationsquarantin...
But the government is breaking up peaceful and legal assemblies. While I don't have the article handy, there have been arrests over the past few days for refusing to leave.
As for "legal" - the First Amendment is very clear on this matter. The federal government (and through the incorporation doctrine, the governments of the several states) is specifically denied the power to prohibit this.