There is more than stumbling in Stumbleupon.com. You can also discover sites, which is more like submitting sites to directory and that in turn stumbled upon by other users, reviewed and the cycle goes on. The more users thumbs up the sites the more it turns back to many users.

Recently i had some problem in submitting a site and found that it happens due to some problem with the installation of the extension. The fix is quite simple. visit this site to re-install the plug-in and after that it should work just fine. This is mainly due to various releases made and the plug-in do not get updated automatically.

Also stumbleupon is much better than del.icio.us, digg and other social bookmarking sites in its working as it brings in random contents of your preference and rated instantly and is a great tool for increasing the traffic of a website. One thing to be noted that invalid contents cannot survive for a long time and also you cannot keep submitting your own sites and blogs as it is tracked based on usage. So all in all dependable.

This is also a great tool for New webmasters once you know to use it properly. Wont work when you keep discovering only your blogs and posts. Users behaviour is in track and stumbling 20 sites and submitting one of your post will do justice.

Well I don't know whether it is in fact bigger then digg.com because it certainly has less users but it depands what kind of content you are searching for because digg.com is more or less just well not just but mostly based on computer sites and computer based things. So I suggest you that you consider also these, and certainly if you hvae something that might be of interest to any computer people groups and various peers then you should ceertainly check it out on the digg.com as in my very personal oppionion it is the best site of this sort.