While Facebook is obviously not bound by the first amendment, the fact that something is paid doesn’t undermine its nature as free speech. Thomas Paine’s “Common Sense,” which sparked the American Revolution, was published and sold for profit. https://www.printmag.com/editors-picks/power-of-print-common.... (Though Paine donated his share to the Continental Army.)

Paine got his message out by contracting with for-profit publishers to disseminate his book. Facebook is just the new version of that.