StumbleUpon is the number one referral source of traffic on the web. The below Infographic shows the difference in stats of webpages get added to it and the stats other Social sites gather in the same time. The time that a link continues to refer visits from StumbleUpon is much longer than sites like Facebook or Twitter. Unlike Facebook and Twitter, StumbleUpon is based on popularity and quality of content. Check out the Infographic describing the Life cycle of a Web Page on StumbleUpon:
Feel free to stumble this infographic!