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First, I would not bet anything on Alexa data.  It is based upon a toolbar that very few people use. If you are interested in what a forum does to your search engine rankings then start tracking your Google rankings with the SEOmoz rank tracking tool. Now, can a forum damage your rankings?  I can imagine two situations where it might. lots of trivial content, duplicate content or trash content is posted in your forum forum members flood your threads with links out to spammy sites, manipulative sites, or sites about topics that google does not like

thanks Marcin - Very helpful of you!  I guess I just had to learn how to use the tools provided. Much appreciated

Thanks Josh, Great help.  I will definitely check out these resources. Appreciate it!