ProBlogger:
The potential for StumbleUpon to send traffic is often under-estimated, particularly by new bloggers. Unlike digg and del.icio.us, an item doesn’t need to become popular before you see immediate results. One or two votes can bring a hundred or more readers - more than a new blog might see in a day.
StumbleUpon users are, however, notoriously fickle. The service describes itself as allowing you to ‘channel-surf the internet’ and I think it’s a very appropriate description. Users flick through websites like you might flick through channels, often making a decision on whether to stay or leave your site before it has even had time to finish loading.
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Many stumblers are stumbling for fun so if they find their ride full of fun they will definitely use the thumbs button . Stumbleupon traffic is indeed a good one for web pages that have interesting stuff .