It's still technically underneath the "logo" for StackExchange... but it's changed location and how the actual button looks like... it's at the right of the top bar, underneath a chat bubble-looking item that is meant to be the StackExchange logo, as shown in the screenshot below (freehand circles and arrows included for somewhat obvious reasons).
Click that icon (it's the one indicated by the arrow), and then you get the popup menu there. The 'log out' button is there. (it's the free-hand circle)
This screenshot is while looking at this exact question directly, here on Meta, but it's in the same place on the main site as well.
