I read on this blog about the author's algorithm for ranking the expertise of a user in a collaborative tagging system, like Del.ICIO.US

http://www.michael-noll.com/blog/2009/06/05/telling-experts-from-spammers-expertise-ranking-in-folksonomies/

I found it very interesting. Perhaps its a little too scientific for the masses but with the correct fine tunning or add-on tool, it will probably become very helpful for businesses. What do you think?

I don't know that you need to go to this level of analysis to tell friends from spammers. The algorithm thing seems like a case of overthinking. It the population you have to analyze is manageable you can simply read their content and contributions to determine the dross from the gold.