’Followers count’ as the default sort is a little tricky because it is still possible to game it, and having this be the default would encourage this kind of gaming.  ’Copies sold’ is tricky because it only applies to projects that are funding or have funded.  The current ’relevant’ sort attempts to track community interest. I’m a little reluctant to say exactly how we’re doing this right now, but I will say that we have some features rolling out in the near future which will shed some light on it.