From the article (by Scott Ambler):

UML 2 activity diagrams are typically used for business process modeling, for modeling the logic captured by a single  use case or usage scenario, or for modeling the detailed logic of a  business rule.  Although UML activity diagrams could potentially model the internal logic of a complex operation it would be far better to simply rewrite the operation so that it is simple enough that you don’t require an activity diagram. In many ways UML activity diagrams are the object-oriented equivalent of  flow charts and  data flow diagrams (DFDs) from structured development.