When does a persuasive pattern become a 'dark' pattern?

The UI Patterns website lists a large number of current and emerging design patterns.

Under the section called Persuasive Design Patterns, and many of these patterns are commonly seen in social media websites that are aimed at attracting new users or creating incentives for them to spend money. It feels like that some of them would fall into the definition of a 'dark' pattern.

My question is: what types of persuasive patterns fall into the 'dark' pattern category, and is there somewhere that is sensible to draw a line?