Social media sites are multiplying like rabbits. A while back, I posted the Alexa rankings for 42 popular Web 2.0 social tagging sites. Later, I wrote about a couple more, and newer sites such as Twitter keep popping up. Wikipedia lists more than 100 social networking sites, and Social Media Trader recently compiled an even more exhaustive list of 115 social news and bookmark sites rated by PageRank, number of inbound links and dollar value of each site.