Compete's numbers are pretty good, you just have to understand the source. Digg showed more traffic, because Compete's datasource is US ISP logs, and any site with a digg button was added to digg's traffic.

The comparing reddit/digg on compete doesn't make much sense because of the digg button penetration. Just like comparing linkedin to google plus is stupid because many google properties make requests to plus.google.com.

Compete is a valuable tool, you just need to learn how to use it appropriately. Learn when it is reasonable to compare sites using it.