Submitting multiple times and extensions of the same url to these websites can be considered spam, but one submission is fine.
To maximize bookmarketing, post a link on each of your pages to increase bookmarks sites. This makes it much easier for your visitors to bookmark your site on numerous social websites. The most popular social bookmarking sites are stumbleupon.com, myweb, digg.com, and del.icio.us. Instead of manually placing links to each social bookmarking site, socialize-it.com allows you to place a small piece of code on your website, which automatically creates links to many bookmarking sites.
Our tests have shown that significant traffic can come from social bookmarking sites, namely digg.com and stumbleupon.com. In fact, our test site went from 220 unique visitors per day to over 350.
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