> The fee is based on the formula (number of users * $0.001), so the earlier you join, the less you pay.
Bing's web cache shows the price being at (least, since we don't know when Bing took the cache) 6.87 yesterday (http://cl.ly/0A1N0K0C2Q2W2H0U3h43) and now it is at 8.35. That is a difference of 1.48, indicating a gain of (at least) 1,480 customers. Every user x signing up past the point that the cache was taken generates (6.87 + 0.001x). Thanks to Gauss, we can find the total revenue using (6.87+8.35)(1480/2), and we can calculate roughly that Pinboard made something on the order of $11k yesterday, and probably a bit more since they probably round up instead of charging tenths of cents. Sweet!
Hopefully this helps the guys over at Pinboard generate positive ROI for their time, since Pinboard is an awesome service.