Amazon is another example. I didn't notice this trend until you pointed it out.
Someone else asked the same question: Why would a web site hide the log out button?
Here's a short post on it that pretty much says:
By hiding the logout feature, you're more apt to simply close the browser or tab, but effectively remaining logged into a service. This allows Facebook to openly track your online whereabouts via advertising partnerships that all report back to Facebook.
Makes sense, as you pointed out that 'Login' is usually still very much visible. Sites like Facebook/Google have the luxury of violating principles (or perhaps, evolving them) because people will grumble but continue using anyway. I agree with user12999 that for the most part, you should probably stick with keeping it visible.
