Social media sites for social media marketers

Social media sites for the social media marketers to consider, grouped by users:
The following recommendations --

100 million+ users
Facebook
Twitter
LinkedIn
YouTube
MySpace

25 million+ users
Reddit
Flickr
Yelp
Wikipedia

10 million+ users
Tumblr
StumbleUpon
Care2
SlideShare
Scribd
DeviantArt
Digg
Delicious

Up-and-coming sites
Posterous
StackExchange
Namesake
Quora
FourSquare
Hunch
Forrst
Dribbble

There are plenty others sites worthy of your attention as a social marketer
Expect even more to appear during 2011

Please add your favourites to this list please