The First Amendment protects citizens from censorship by the government. It doesn't protect the government (or citizens) from censorship by private business. This is why having unregulated private platforms be the primary forum for political discourse is a threat to democracy.

If Facebook or any other social media site wants to do that, they should be classified as a common carrier and be regulated by the FCC.

"One reason common carriers are licensed is to ensure they provide indiscriminate public access and protect the privacy of the general public."

https://whatis.techtarget.com/definition/common-carrier