Can someone direct me to a code sample of how to use libpurple from .NET?
libpurple is an unmanaged library - you'll need to either obtain a wrapper for it, or write your own. Quick Google search did not reveal any good .Net wrappers so writing your own might be your only option.
Here's a simple tutorial on writing a wrapper: Call Unmanaged Code. Part 1 - Simple DLLImport