yeah Compete is just crazy+depressing.
For Mibbit it shows a downward trend for the last 12 months:
http://siteanalytics.compete.com/mibbit.com/
However, the actual data, and the quantcast version is an upward trend, with traffic doubling:
http://www.quantcast.com/mibbit.com
Maybe compete have got some issue with all their estimations that means everyones site has dropped over the last year. I don't trust anything from compete though.
Quantified is definitely the way to go as long as you don't mind having the exact stats out there for all to see.