A shade different than the social networking sites just mentioned, social bookmarking sites are merely user-submitted links

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to search-based categories, such as Twitter, Reddit,
Digg and StumbleUpon. Which admittedly sounds very simple, and it is.
All you have to do is join one (or five) of your choosing, and then submit links of things (or comments about links of things) to the database. If other users like your suggestions, they'll vote positively, and it'll move up the rankings.
So, although you can't do this exclusively for your small business (users figure out pretty quickly those who are merely spamming the system with their own small business links) if you actually want to have a discussion with your customers about related products and services, this is an excellent way to garner new, interested and buying customers without a lot of work.
For more information: Using Digg and Stumble Upon To Attract More Customers, and How To Use Twitter For Your Business.