I can see good reasons for Facebook to build a phone, and would bet even money that that's what they're doing right now. However...
Media: "Facebook is making a phone!?"
FB: "No, absolutely not. Definitely not."
Media: "Well, that's exactly what you would say if you were making a secret phone!"
FB: "..."
I think Facebook is likely at least considering making a phone, and mocking something up in its skunkworks. I don't know that it would be very good, though. Only time, or lovely leaks to Palo Alto bars, will tell!