Multiple groups or one group per category

I am building a social networking site which will allow users to add themselfs in group. Now, I am in delima to allow users to create groups or not.

• Single group concept - StackExchange, Quora etc.
• Multiple group concept - Facebook, Google+, etc.

Now, the question I have related to it:

• Do user feel comfortable joining multiple groups for the same category?
• What is the benefit of choosing one concept over other?
• Is there any criteria for choosing the concept?