Help on exit
Most of the online services I use put the "Sign out" link at the end of a drop-down list in the top right corner of the site. This has become a useful convention that allows everyone to just click this link and leave the site without bothering what the rest of the links are about and without giving further explanations. But Facebook thinks that it can violate this convention. So the last link in their drop-down list today is "Help", not "Sign out".
As a result, people who are used at clicking on the last link will get help each time they want to leave and the site will behave as if they aren't proficient enough to know how to do what they want. Not only is the expected action not happening, but something else is loaded instead. From a design standpoint, this is not the right way to increase the average time spend on the website. If it is hard for people to leave, we can expect that they will be more reluctant to come back.
In order to work, design shouldn't be confusing. We can think that we are more clever than the established conventions, but we are wrong. They were formed slowly over time as a result of the experience of many bright people working in different circumstances and on various projects. These people came to similar conclusions independently of each other, which is what makes these conclusions so powerful. It becomes a design problem when a single company with lots of designers tries to consciously impose its viewpoint on the world.