This survey of professional bodies in 2011 indicated what they are using social media for. It seems that You Tube is most used.
Here are some examples:
This CIPD linked vox pop on ‘What is the secret to engaging staff’ was interestinghttp://www.youtube.com/watch?v=8bthRxOHlP4
The Big Society: Challenges and opportunities for membership organisations (RSA). The RSA, The National Trust and NCVO views - http://www.youtube.com/watch?v=yxxR4o66Y7o
A successful ad campaign for British Association of Dermatologists -http://www.youtube.com/watch?v=tJjbTkk8tKo
And finally a great ad with over 1,000,000 views http://www.youtube.com/watch?v=MnSIp76CvUI. Not membership but it’s gone around the world virally.