If you are looking for sites that can help you with social media marketing then check out the likes of:
www.viperchill.com
www.pronetadvertising.com
www.sphinn.com
If you are just looking for traffic and member figures check out the likes of:
www.alexa.com
www.comscore.com
www.compete.com
www.techcrunch.com
Hope that helps
Cheers,
Glen