More Than 1 Million Members

You read my headline and I bet you are as interested as I am, well at the moment I am more curious on how on the earth a community can have more than 1 million registered users and I have not heard about it before today? I am not sure if my information is correct, but I heard that StumbleUpon has more than 1 million registered users. That´s just awesome, and what a great name

At the moment I am thinking should I join or not? I have just joined MyBlogLog and I am very satisfied with it, now there´s a new community to me, this would probably be a competing community and I am not sure how I should manage to join both. It might hurt traffic more than gain anything from it. Well, I am not sure… but it looks really interesting and I would like to test it.