Essentially, StumbleUpon is a social or community bookmarking provider. It is also one of the most popular
social bookmarking
provider currently.
What is Social Bookmarking? Well, social bookmarking is actually a process that allows you to bookmark sites or web pages like what you did with your Internet Browser like putting them into 'Favorite'. The only key difference is that, social bookmarking allows you to save these bookmarked sites or pages on the provider's site. There is also an option of making it 'public' or 'private'. Public means anyone can see it while private is vice-versa.
What is the advantage of bookmarking then? It is really effective in bringing traffic to the site which are being bookmarked in site like StumpleUpon where online community members will be brought to your sites like either searches or 'Stumbling'.
StumbleUpon There is actually a feature with StumbleUpon really fun to use and that is the 'Stumble' button. A click on the installed tool-bar into your browser will enable you to randomly being brought to members' sites or pages. The installed tool-bar also allows you to rate the pages that you have visited, to be good or bad. If you like the site, a box will popped up and you just fill in the description if need be. You can also rate your own sites to increase your site's traffic.
Based on my personal experience, StumbleUpon is really fun to use and also an effective tool to bring in traffic.