The internet is HUGE. And when I say HUGE, I mean it.
According to Wikipedia, there are currently over 100 million websites on the net ( More are getting added as you read this post ). 
While the figure of 100 million is good to read, one must remember that a good amount of those 100 million consist of sites that have little or nothing for the end reader. Most of these are spam, created with the sole reason of boosting search engine rankings for other sites. And there are many brilliant sites that are not known to all. Why ? Because their owners do not really have the time for promotion, and word of mouth is the general way that people come to know about these sites. And here is where social bookmarking sites come in.