Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
The logout link is now missing for Stack Overflow as well as meta. Haven't check out the other sites.
Any particular issue here?
To supplement Rebecca Chernoff's answer:
The link has been moved to your user profile. Click on your username at the top of the page and you will see the link there.
<a href="/users/logout?returnurl=/">log out</a>