I have some doubts... The people you would attract with a social media module already use social media like twitter, linkedin and facebook. These are already the places where people unite in groups, have their discussion spaces and so on.
I guess it would be better to make modules working together with existing social media, make your site part of it rather then build our own limited facebook clone. Give all modules share to facebook, G+ and twitter buttons would be a good start, using a facebook api for registering or commenting on your site and so on...Extending the user profile with some nice functions can be part of that strategy.
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