> Censorship, the suppression of words, images, or ideas that are "offensive," happens whenever some people succeed in imposing their personal political or moral values on others. Censorship can be carried out by the government as well as private pressure groups. Censorship by the government is unconstitutional.[1]
Facebook has enough power over communication at large to effectively suppress speech should they choose. Censorship does not have to be perpetrated by governments. Hence the terms self-censorship and corporate-censorship[2].
What people really mean when they talk about censorship is first amendment rights. Here, you are correct. The first amendment protects against government censorship only - nothing Facebook does could infringe your first amendment rights.