Pinterest's meteoric success is bafflingeven now, nearly three years since it was launched in March 2010. We've said it before and we'll say it again: Pinterest "channels and encourages the most regrettable impulses of the social web; and siphons content from creative enclaves while offering almost nothing in return." That it has become so beloved by so manyit has over 20 million unique visitors a monthis surprising, especially given Facebook's social dominance.