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You are right the numbers are only an estimates but I disagree with you that they are "Worthless", is always good to have an idea of your competitors traffic.  You can see what is the potential of market x or y looking by looking at big players estimate traffic.

Hi Leigh. This will depend on a number of things. How old is the site? Have you been link building? What else have you done in terms of SEO? If the site is new and you have only recently begun working on the site it may take a month or two for Open Site Explorer/Linkscape to pickup your site.