[Treborika]

This is because of the following reasons:
-When the forum is of great interest
-When the discussions and topics are correlated to the members own life experiences
-When the moderators are are firm and fair.

[Netherrealmer]

People are not Kumbaya and have diverse opinions. Someone disagreeing with me is a good sign a forum is alive.

[Mika]

Most of the forums that are big are offering incentives to the users. There are few exceptions like Reddit and Bitcointalk.

[augusta]

A multiplying forum is one that is having enough traffic and users to make the site active