I created an account and logged in and asked a question from my laptop at home. While at work, I received an alert, which I viewed on my PC at work. I wanted to add a comment but got a message that I could not unless logged in, and the only login options were Facebook, Google+ etc, and not the username and password. Why it is not top right like the vast majority of sites is a bit of a mystery, so I would appreciate if someone could point me in the right direction.
At the top right is a little logo that looks like a speech bubble. If you click it, it exposes a drop down menu. The first row will include a link called 'Log Out.'
On mobile, this is nearly the same, except that the link is on the far left of the header.
Logout is on the top right at stackexchange.