Paul adams has compiled a very interesting slide deck about The Real Life Social Network in which he introduces a concept called "temporary ties".

Strong and weak ties are not enough when we think of relationships online. We need a new category of tie, and I call it the temporary tie. Temporary ties are people that you have no recognized relationship with, but that you temporarily interact with. 

Once the task has been completed, temporary ties are unlikely to interact again. You don't know these people beyond the one conversation you had, or the words they typed and whatever online profile they have. Your interaction with them is temporary. With the rise of user generated content online, temporary ties are becoming more important.

Read more from Paul's blog Thinking Outside In.