Also: the government could always ask your friend at the other end about what you said, not just Facebook.
The intention of the founders re: the 4th amendment was not to protect communications between people, it was to protect people from personal searches. Read the text: "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures."
Persons, houses, papers, and effects? What do all those things have in common? They are personal to you. You'd feel violated if someone else entered your house or rifled through your effects. That same reasoning doesn't apply to something like a Facebook message that any of a number of people at Facebook could read. Maybe you assume that they wouldn't do that, but they certainly can.
Maybe we need a law to keep the government from getting Facebook messages without a warrant. But that's a separate thing from the Constitutional issue. Personally, I don't even think we need such a law. If you're okay with Facebook or Google looking at your communications, you should be okay with the government doing so.