Paul, you answered your own question with your post. Vibrant, fresh content from a growing group and variety of contributors with your sharp eye for spam and self-promotion and you end up with what you now have-a successful group.
One of the key assets the group has going is it's name. "Social Media Today" might be one of the most keyword rich packages out there, which helps Google etc.
Another reason for your success might be the description in the Group Profile. It reads:
"Theres a lot of buzz out there about Social Media Today Join one of the web's most established and best-regarded communities about social media and its implications for your business, your life and your planet. Also find us at socialmediatoday.com."
Very clear, concise and inviting.
Now take a look at The Customer Collective name and group Profile which reads:
"A moderated, business conversation with the world's leading bloggers on sales and marketing strategy."
To me, not as compelling, the group name does nothing to indicate what the group's purpose is about, and no link to excellent website.
Just my perspective, can't wait to read what others have to say.