The additional 'k' was inspired by digg.com - it wouldn't be as cool if it were called dig.com, would it?
Bio and I took around 2-3 weeks before settling on rankk - we threw up lots of ideas but none could quite describe the site without being overly descriptive.
rankk is shorter than pyramid and is easier to remember and type. It's also indicative of gaining ranks, levels. A name with pyramid in it may be associated more with pyramids than with games. worse, it may be connected with pyramid marketing.