Not official, but a good compilation by our friend, Jeremiah Owyang at Forrester, this post gives a fascinating rundown of how many active users the big social networks are getting.
Jeremiah gives a bit of a disclaimer on interpreting the basics:
"To be clear, my employer Forrester doesn’t provide specific numbers about social networks like Compete, Comscore, Nielsen or others, we conduct our own surveys on user/brand behavior, opinions, and techn...