StumbleUpon has announced
that is has reached the 20 million member milestone. They site can drive a lot of traffic with that many members. StumbleUpon says, "it's very exciting for us to reach such a milestone, all from the simple idea of 'click a button, find cool stuff.'"
StumbleUpon also shared an interesting infographic that shows its path to 20 million stumblers. StumbleUpon has doubled its user pase from 10 million to 20 million in the past year. You can see a larger version of the graphic here